Ninety-Nine Contraceptives In A Box, Take One Now, Then Let's Fool Around.

One, two, little sister is yearning for you.
Three, four, quickly she'll open your door.
Five, six, she needs that fix.
Seven, eight, you're bound to be caught at this rate.
Nine, ten, what does she mean by "ready to go again"?!

Eleven, twelve, she knows that she's got you all to herself.
...
Nineteen, twenty, under the same roof, opportunities are certainly aplenty.
But then there's thirty, when mom and dad finally start to notice that their son and daughter, on occasion, seem kind of flirty.
What with forty? Fleeing home together may become necessary rather shortly.
And fifty, needing to live life very thrifty.
Then there's sixty and settling in an entirely new city.
Fine with seventy, finally living as husband and wife under a new family identity.
Now we see eighty, little sister's seems to get nauseated every morning lately.
Skipping ninety and again straight to one hundred, when she gave you the news, "That's wonderful!" was all you said.

Mother and father shall never meet a single grandkid, not after the things they said about the relationship you two hid.