Thereasons for waiting to "tell" seem valid. You are, of course, in shock. If your sister can wait until after the Holidays why not until your brother is out of school. If he is a senior it is not that long.
It's okay to still love him…but you con't have to even try to understand the abuser part on your own.
If your wife has depression why don't you seek out a good family counselor for your family. You need it, your wife may need it, later, if not now. You also need to strongly urge your sister into counseling now. NOW NOW!!!!
Don't cross any bridges (what is going to happen) until you come to them. You have not heard the "other side" of the story yet either. You don't know for sure what steps your sister is going to take.
What a case for living and loving your own personal family day by day and appreciating them for all the little things that are positive.
I am in no way saying abuse should be swept under the rug….but don't even think about being the judge and jury until it is time.
Good luck. Keep us posted.