If you intend on sending your sim off to uni i would recommend turning aging off since your concerned about younger sibling gaining skills as there's no other way around it unless your sims stays home while going to uni.
Your teen will lose her home if you intend on having her go to school from campus since going to school from home is an option i'd suggest that if you don't want to lose your home.
For a first play through i'd suggest 2 or 3 classes till you get the hang of it or a routine down. I'm able to do a full class load and still have her party/have fun but it also depends on the class schedule and I've just gotten lucky with my sims schedule and gotten a good routine down to make it a good balance.
As for sending a group together i would make them all into one household first and than have them all apply so they can move together.