Just wanted to mention.. on the flipside.. think about how it would affect your relationship afterwards.. my SIL had her 2 brothers in with her!! (ick! if you ask me!! LOL) But I'm not so sure they wanted to be there.. coz if you even mention it they get all embarressed and dont want to talk about it. It's like they're disturbed by it, and I'm sure they've never looked at their sister the same since! So definitely discuss it with FIL, make sure its something you both would be comfortable with.

Sounds like you have great inlaws - I love my own mum but there was no way she was allowed in to watch Personally I like it very private. But just decide how you feel. The escape clause idea sounds good