Well, if that's really what she said...

Sister loses the bet,
You take her to bed.
Sister wins the bet,
She gets to choose whose bed it is instead.

Now, what would you bet,
That "Take me to bed,"
Was what she actually said?
That only tears of joy and ecstasy were ever shed.
And that in all the years since there has never been a single regret.
That in spite of the ties we cut, on the day we fled
To be together, under false names and hoping perhaps for even a chance to wed.
That although at times it was difficult, that life we led
We still always managed to stay together and we always kept our children well-fed.
And it all began, when we both decided our true feelings for one another were something that needed to be said,
Regardless of whatever trails life would then have in store for us both just up ahead.