Few elements is not big deal in the overall scheme of things but his thought process is a red flag and should have you concerned.

It's a wedding for your sister. This is her day, its your family, he is a guest, and you are there to celebrate the new couple. He should stop trying to get attention and make things about himself. It's very selfish. It would be like you or another woman showing up to the wedding wearing a skimpy or revealing dress that takes attention away from the bride.

There is a time and a place for things -- trying to getting attention and using clothing/ accessories to let everyone know what your all about (look at me and how cool I am) at someone else's wedding is not the time or the place. Again -- his proposal doesn't sound crazy but the selfish thinking should have you concerned.