You're never going to make everybody happy obviously. It is rec ball, and as you said in a previous post "It's about the least competitive league you can imagine". My guess is most of the parents would probably prefer to lean to the side of groups of friends playing together than trying to make the most even teams possible. I'd say try to spread the pitchers and catchers around as best you can while honoring the sibling, coach and friend requests.
The best thing as has been mentioned, is tackle it from the standpoint of holding clinics for players and coaches. My DD plays rec and what I've seen is a lot of the coaches don't know how to teach pitching. So have a few clinics to get everyone started on the right path, and then it's a matter of practice. You're not going to develop them into TB pitchers, but for rec, you just want the girls to be able to throw strikes. Speed and the different pitches aren't that important. My experience is girls who are willing to put in the effort, even just 50 pitches every day on their own, become decent rec pitchers.
Yup, on my list of stuff to make happen. The funny part is that the girls all get along, pretty much regardless of the teams - but the parents, well that's another story lol! Some of these groups act like it is TB.
My concern is exactly pitching and catching at 12U.