It sounds like you have an instinct whether it will be good or not. Follow that instinct.

Try to follow-up on the pros and cons, give them weight and weigh them.

Don't be afraid to make 'a wrong turn' just course correct as you go.

Sometimes, you have to rely on a short-term solution (lesser of two evils type of analysis).

If at all possible, it may be best to keep gifted siblings or siblings of a gifted student separate for many reasons. Overall, it can be much less stressful. Good Luck.