Aw, poor you. I noticed you said you are about to have your own baby? You must be hot and heavy and hormonal and fed up. I remember one time I slid down a wall and sobbed for ten minutes because we'd run out of something. The last few weeks are not a woman's most reasonable time. This lot are being a bit hard on you I think!
I also think that your own brother surely has a hand in naming his own child? If he knows what it means he can talk to his wife. He certainly could have said to her how much that name meant - so perhaps instead of blaiming her alone you could remember that your own brother has gone along with this and HE is supposed to care more than your sil!!! So where is your anger towards him??
I think you really need to change the way you interact with her. STOP telling her stuff like what you are going to buy, or your redecoration plans! Or at least be vague - or tell her lies, like you are going to paint the walls bright green and have a tangerine coloured sofa And stop telling your brother this stuff too - since he will obviously tell her (quite reasonably since she's his wife!). Create some distance.
Or even start saying "We got X from such and such shop, I know you tend to like to do the same, so I thought I'd let you know they've got a sale on" and all sorts of stuff like that. Might make her think?
Finally...take a deep breath. What matters here is YOUR family. That's you, your husband and your baby!!! Make them your focus, don't waste your time and energy on outsiders (because BLOODY HELL you are going to need to save it for that baby! )