The wedding happens relatively quickly, within a few months or so. True to her word, Celestia helps plan the rehearsal dinner, and goes absolutely overboard. At Twilight's and Flash's insistence, she sticks to their own, vanishingly-small budget, but her desk is thick with pamphlets for caterers and patissiers for weeks. In the end, Celestia is able to stretch their budget absolutely as far as it will go, and manages to furnish a legitimate three-course meal on just a few bits. Though Celestia is quite proud of what she's done, she's nervous as a schoolfilly to see what her adopted niece and nephew think; turns out, they're genuinely pleased as well, and both thank her seperately for all the effort she went to.
The ceremony itself is a relatively small, semi-private affair--Twilight's friends attend, as do the Sparkles, and Celestia, Luna, and Cadance, but very few others. Celestia can't help but notice just how much Twilight glows during the ceremony, and Celestia likes to think that, maybe, she's had a little something to do with it.
Twilight is still in school, but Flash works hard to support the two of them (with more than a little help from Cadance, who he still works for). All too soon, they add a child to the family, a tiny filly they name Aurora. Celestia comes as soon as she's able to get away, and, though she initially has that same sort of disconnect she had around Flurry Heart, she is able to force her way past it and become a smothery, blubbering, embarrassing mess. Depending on the timing, the two of them may even have another child before Twilight graduates from Crystal U.
(During this time, Flash reveals himself to be a rather private pony. He's rather quiet and humble, and tends to keep a tight lid on his feelings, which makes the few times he does get emotional [e.g., at the wedding, at the birth of his kids] all the more impressive. He comes to relax a little more around Celestia, but still prides himself on his stiff upper lip. However, he is a very good friend, and becomes close with Rarity in particular. The two of them often have friendly little spats over their different ideas of "refinement.")
Upon graduation, Celestia offers Twilight a position, but she's determined to make it on her own, at least as much as she can. She looks for a job for a while, until she suddenly hears that Cheerilee and Big Mac will be leaving Ponyville (perhaps to help out the schoolhouse in Appleoosa?). Despite Celestia's somewhat-desperate offers of similar positions at her own School for Gifted Unicorns, Twilight insists on moving to Ponyville--she misses her friends, and wants to be close to them again. Nevertheless, Celestia is able to pull some strings, and Twilight is able to move back into the Golden Oak Library, and take the low-stress librarian job in what free time she has. Flash is quickly hired by Applejack, and he takes to the farmhand life rather well.
Twilight draws closer to her friends again, and, though all of them are busy, Twilight organizes little get-togethers whenever she can. As they each get married, they begin to bring their husbands and, sometimes, their babies--and Celestia is always invited, of course.
And then, to everyone's astonishment, Rainbow Dash suddenly and unexpectedly dies. Rainbow had gotten onto the Wonderbolts Performance Team--her life-long dream--but had been hiding a secret. Years ago, she had broken one of her wings, and, though it eventually healed, she flew on it again before it was completely better and re-injured it. During practice, especially during the more difficult maneuvers, that wing had started to hurt, but, fearing for her position, Rainbow said nothing.
No one knew about Rainbow's injury--until, in the middle of an important show in a packed stadium, right as she was in the middle of a complicated routine, her wing gave out and she plowed into the stands.
Celestia pauses, then tells her listeners that Rainbow Dash apparently died instantly. She was one of the very lucky, very few, who could say that she died, literally, doing what she loved.
Everyone attends Rainbow's funeral, and make a big ol' fuss about her, the kind that she would have loved in life. Each of the friends reacts differently to this, their first loss among them--but Twilight, especially, is deeply shaken. With Rainbow on the road so much, Twilight hadn't seen her in far too long, and, even more than the rest, wasn't ready to say goodbye.
After several weeks of deep thought, Twilight starts a mailing list. Her friends can send her letters, and she'll magically make copies, then send them out to everyone on the list again. Initially, the list is just for the five of them, but it soon expands to include others affected by Rainbow's death, including Scootaloo. Slowly, it grows until dozens of ponies are writing and receiving letters. Celestia is a frequent, though anonymous, contributor, and uses the name "Heliotrope" (i.e., "sun-turner," a pun she's rather proud of) when she writes.
(During this time, Twilight and her friends do a little experimentation, and realize that Flash Sentry can actually use Rainbow's Element of Loyalty. However, it's just not the same, and Flash, as always, is a little uncomfortable with the attention.)
One day, Twilight gets sick (kid #3 or 4?), and Flash ends up substitute teaching. Though he's reluctant (claiming, once again, that he's not smart enough for this sort of thing), he discovers a real talent for teaching history, in particular the Pre-Classical era (which he was actually alive for). From then on out, Flash eagerly subs for Twilight whenever he can, and, eventually, he's able to get a job team-teaching with Twilight--she handles math, science, and magic, he does P.E. and English, and they both split history. Twilight is an amazing teacher, kind, patient, and helpful, especially with the more difficult subjects, and Flash is very genuine and approachable (eventually, he buys a bowtie that both Twilight and Celestia absolutely adore on him).
Twilight also gains a reputation for being especially wise; whenever someone comes to her for help--especially for a problem between friends--she's known for sitting them down and working through the problem with them, and teaching them what they need to do to fix it. Twilight doesn't talk much about this aspect of her work, but, when Celestia accidentally catches her doing it, she is immensely proud of her--Twilight is putting her Friendship Reports to serious, practical use, and doing her best, as always, to help those around her.
Despite the distance, Celestia visits Ponyville as often as she can, at a minimum of once a month. She gets into the habit of leaving her regalia at home, and making the guards stay outside on the front porch--when she's with Twilight's--no, with her family--she is glad to be simply "Aunt Cece." She comes to spoil their many kids--and, eventually, grandkids--absolutely rotten. However, her favorite time of day is still after all the little ones have been sent to bed, and she, Twilight, and Flash can just talk.
Cadance, too, continues to have children. About a quarter of them, all girls, come out Alicorns--and, though they, like all kids, have their rough spots, Cadance proves to be an absolutely perfect mother, with kids to match. "Granny Luna" visits them fequently, perhaps even more than Celestia visits Twilight, until it becomes something of an open secret that their relationship is more than they let on.
Celestia continues to visit Twilight whenever she's able--but does her best to ignore the growing wrinkles on Twilight's face, and the little streaks of gray in her mane.