I never thought about this particular aspect of having boy-girls twins until last year at a Gemini Crickets general meeting on kindergarten readiness - the tendency for the girl in the boy-girl twin relationship to "mother" her twin brother. Being a twin myself (I have an identical twin sister), I have sort of counted myself lucky in having boy-girl twins, because my sister and I definitely dealt with identity issues growing up. Being called "the twins" or "the girls," our aunts and uncles not knowing which one was which, being compared by family and friends (which one is smarter, prettier, a better eater, more coordinated, etc.) - despite everyone's best intentions, some of this is kind of inevitable. My sister and I are very, very close now, but went through a period of time where we consciously tried to be very different from each other to sort of find our own identities. This was, of course, during high school - when everyone tries to figure themselves out!
Anyway, I thought that the identity and dependency issues that twins often face would be less, if not non-existent, for boy-girl twins. After all, boys and girls can be so different from each other. They certainly look and dress differently, are often interested in different activities, naturally gravitate to having different friends, etc. So when the kindergarten readiness speaker was discussing whether or not and when to separate twins into different classes in school, I was surprised to hear that sometimes boy-girl twins do better being separated because the girl will tend to mother the boy, such that the boy really lets her do the bulk of the work and take care of him.
Looking at my own 4-year-old twins, I can certainly see that this is happening and has been happening for a while. My daughter loves to be a "little mommy" to her twin brother (and to her little sister too). And my son is perfectly happy to let her do all sorts of stuff for him. Take, for instance, today, when it was time to get dressed. My daughter got herself all undressed and dressed. Easy. Done. My son....sat on the couch and let his sister pick out his clothes, and start dressing him. He happily soaked this in. She happily played the mommy, parroting all the things I usually say during this process - "Do you want to pick your clothes today or should I?" "Pants down, please." "Arms up, please." "Go pee-pee in the potty, please." "Try to put your clothes on yourself, please." "Ok....all set! Good job." Although I want my son to learn how to dress himself - someday...please? - it sure is cute watching the little mommy in action. Luckily, the mothering hasn't gotten out of control, but I'll definitely be watching for it.
I am sure this can happen the other way around in some cases, where the boy is more aggressive and the girl more shy, so that the boy takes the lead. I guess it always depends on the particular dynamic of your twins. Personally, my kids are doing just fine in preschool being in the same class - although they are in different group times - and I will be keeping them together in school as long as there isn't a problem. The research into whether or not to separate twins in school covers many issues, but I basically believe that twins benefit from each other's emotional support by being together in the early years of schooling. And I have learned that although many schools will have a policy of separating twins, you can often prevent this if you feel strongly about it. Things can - and will! - change over time, so I will see where my twins are at when it closer to the time for kindergarten. In the meantime, my "little mommy" will be helpful in all kinds of ways. Maybe she can start putting her brother to bed and making him breakfast/lunch/dinner/snack. Better yet, she can teach him how to cook for all of us!