To update everyone and thank you for your advise.
To summarise the last few weeks, my husband phoned his sister in the end and asked her if she really wanted us all there. It was a bit stilted, but she said yes we'd all been invited. He told her that the children are lively and we didn't want to ruin her 'special day'. She still said we were all invited (in a 'huffy' way my husband said) and there would be other children there.
We went, parents-in-law very pleased, all other relatives and even the bridesmaids nice, friendly and kind about the the children (saying how lovely they were etc.)
We went up to sister-in-law and her new husband to congratulate them, sister-in-law said about 5 words (I said how nice her dress was, she said something about dieting to get in it). Didn't get to introduce the children as she then went off, pulling husband who said "I think they want us upstairs", as he was pulled away!
Sister-in-laws' husband seemed confused when my husband spoke to him later and said something like "you are now my new brother-in-law". He looked looked oddly a couple of times during the meal! I went at the end to say we are leaving thanks for having us to the bride and she couldn't even look at me!
My eldest realised what was going on and was a bit quiet during the day, but liked seeing all the other relatives who she knows and meeting those she didn't. My other children didn't seem affected by it at all and seemed to have a nice time.
After some thinking about it all. We had a nice day and everyone but the Bride and groom were pleased to see us; and kept saying how lovely it was to see us and what a lovely family we have etc. So it was nice in the end.
However, I think sister-in-law was made to invite us. She should have said to my husband when he phoned her, that she'd rather we didin't go.
I'm glad its all over now!