For those who find BT involvement with their non-frum families to be problematic –
In most cases what you find unacceptable or shocking is something that has probably been brought to a Rav for advice as to what can or can’t be done. Most BT’s I know have a Rov who is knowledgeable about these issues. In many cases the Rov permits, for the sake of keeping good-will in the family, participation that would not be permitted in other circumstances. A BT going to a sibling’s wedding in a Conservative synagogue is not in the same ball-park as you, an FFB, going to your second cousin’s wedding in the same place.
The question here is one of maar’as ayin – yes it looks fishy. But the real issue is dan l’chaf z’chus. Assume that the question was taken to a Rav and his advice followed. If you’re not a BT, you have no idea the conflicts and heart-break that some go through in order to be shomer Torah u’mitzvos despite misunderstanding and even outright oppostion from their families. Ask yourself if you could pursue your own course when every single member of your family thought you were crazy? Those of us who don’t have to go through this diplomatic nightmare should be grateful, and give the BT credit for his/her choices and fortitude.