Not sure the OP will be back. But we do need some things cleared up. She indicated the family is close so I'm just going on that. Let's see if she fills us in on the details. Right now we don't have much to go on.
I was talking to my daughter last night about this very thing, (I refered to this thread ).
She told me some very interesting things that I didn't think of.
She is the older sister and I going to get married in November. I told her I would never given it a thought about who she and Marcus chose as bridesmaids and groomsmen.
Any way she said that when she was still at home, she would be going to do something of a social event, and that Mom would try to always get her to take her brother with her, and that even though she is close with her brother, she just wanted to go be herself, and have her own fun.
I think as parents we do this, often not thinking but there by robbing the older of their independence and freedom by thrusting the siblings upon them.
So I'm thinking that the fellow just wants to have his independent life, and not be responsible for the siblings for once.
There is a lot to family Dynamics.
I have a some philosophy in the matter. I let everyone who wants too participate with me and at their level. I have just a few ground rules.
1. I don't go stupid places to do stupid things with stupid people.
2. If I have plans you can join but the plan isn't changing. They can be modified to include, but the plan is the plan.
3. Appropriate to the situation behavior is expected. I don't tolerate drunkenness, drug use, overtly foul language, or disrespect from or toward anyone with me.
Otherwise come along join in be a part have fun.