MrsBoo, YANBU. My cousin really upset me by having my sister as a BM and not me too - bit pathetic really, as we were in our 20s at the time, and my sis & cuz had both been at the same Uni at the same time together so were close to each other, but cuz & I had been pretty inseparable as teenagers, so I was really hurt and pretty gutted actually.
For my own wedding, I had 6 bridesmaids ( it was going to be 7 but I had to 'let one go' - and that's another story for another thread...). MoH was my sis, and BMs were my two (adult) BFs, and all three nieces on DH's side. We also have 8 nephews (!!!) so we decided pretty early on that unfortunately, there was no way we could include all the lads in the ceremonials, but the nieces were a given from the word go.
DN1 was 14 at the time; she had never been a BM, and was so desperate to be ours she hacked into BIL's email account and emailed DH to "suggest" it . Of course we were going to ask her anyway, and there was no way we would have left out our other DNs. DN2 was 8, and DN3 was 6, and they were all so thrilled to be BMs together, it was so lovely.
DN3 also has SN (dyspraxia, speech problems, ADHD) and had just had her DX a few weeks before our wedding, but she was so beautifully behaved on our Big Day - and even if she hadn't been, I wouldn't have given a f*ck because she is my lovely little niece.
So in short - YANBU. Your DH needs to have a word with your SIL, and your little girl needs to feel included in some way even if she can't be an "official" BM. SIL needs to get a grip and realise that little children can definitely rise to the occasion when required, and that the pleasure of including them is totally worth it.