To raise to 50% the chance that each child will have a sister (or in other words, to have two daughters), you need to have three children
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Every time a parent tells me they're going to have another child because they always wanted to have a [gender], I always get this image of them standing at a craps table, shaking the dice fiercely, and saying, "Come on, [gender]." Usually, followed by, "Doh!"
I did not go around telling people I wanted a boy or a girl. The last part of my earlier post was partly in jest.
I'm not saying you did, and my post in response is entirely in jest.
But giving your child a lifelong friend is a good thing to do if possible. My parents are getting old, and they don't want to leave the city where they have lived for decades and accumulated many friends and join us. They recently bought a condo in the same building as my mother's younger sister, and having her and her husband around really boosts my parents' spirits.
Sounds like that's more of a benefit for your parents than for you as a sibling... not that there's anything wrong with that.
An earlier poster's comments about siblings not getting along well into adult life jibe with things I've seen, too. Similar genes and background are no guarantee of friendship. Sometimes it blossoms into lifelong hatred instead.
