C.H.
I'm not sure where to start, but I absolutely know where you're coming from. I am actually on the other side of the problem though. My mom has adored my son Jacob since the day he was born and brags him up all the time (especially to one of my sisters, who has 8 kids herself). I'm sure it gets extremely annoying to her too. Luckily I don't see you blaming your sister for the problem, because I'm sure she probably has nothing to do with it. I would guess that your mom has just formed a special bond with this child for some reason like my mom did with Jacob. I can pretty much tell you that for sure because my mom doesn't seem as close to my younger son. I know, it sounds awful! She loves all of her grandkids though with all her heart... I think they just have a special attachment sometimes, so I'm not really sure what you can do about it. I guess you could mention the problem to her, which is better than not talking at all, and hope that fixes things. Maybe she doesn't realize she's doing it. Also... if she sees this nephew a lot more than you, that may be the reason for the bragging... maybe she's just filling you in and you're taking it personally.
Hopefully you're letting your mom visit your kids frequently... even if you don't let her babysit, it's a really good thing for them to have a grandmother there to spoil them. Good luck with everything and your feelings are perfectly normal, gloat about your kids all you want.. I'm sure they're amazing. (Sorry for the book)