I think you should def discuss it with him... just because SIL said he wanted to see a birth doesn't mean that is exactly the case. It may have been a wish or passing comment but he actually might feel uncomfortable being there IYKWIM.

Otherwise I think its totally up to you - if you're comfortable with it then go for it. I think its always best to warn support people that when things actually get going you may change your mind (you never know) I told both my mum and partner this LOL because I didn't want them to get upset if I suddenly decided that I was in too much pain and couldn't handle them being there or something like that! Doh - just realised that Dachlostar said exactly the same thing.... I should read these posts more carefully sometimes!!!

Let us know what you decide!