Maybe you're right, Esmerelda, but I'm sure the op is reading and thinking about our replies. Some people take a few days to think things over before they come back in and answer questions.
I don't see where the one dd asked ohmother to speak to her other dd/the bride about including gd in the wedding party, just that the other dd/the mum "grumbled" about the situation. Ohmother might feel like "piggy-in-the-middle" but she doesn't have to be.
Please don't let yourself get drawn into that, ohmother. And don't openly agree with your other dd or try to defend the bride. Either could easily backfire on you. It may be easier said than done, but please try just to listen to you other dd and maybe make some sympathetic replies like, "Aww" or "I'm sorry this is upsetting you."
Your dds are both grown woman, surely, and know how to talk with each other if they want to. I don't the other dd/the mum should say anything to her sister, but that's up to her. In the end, it's the bride's decision who she has for bridesmaids, etc. She knows she has a niece, so if she hasn't asked her, I'm sure she didn't choose to for whatever reason. She didn't "forget," etc. Please just stay out of it and enjoy the wedding.