Fast forward to the holiday break and while out with her mom in town again, apparently there was conversation about my gf's younger brother proposing to his gf soon and he was looking at rings that "resembled" the one I already got for her, which made my gf very upset

Why did it make your gf upset? Because her younger sister might get engaged before her? Or because teh ring ressembles hers (which is not too hard)?

but her mother is making it seem as though this is not the case. If it were up to me I would spill the beans on everything, but I also want this ring to be a surprise if it can be.

Sit your gf down, tell her what her mum is doing, and rediscuss your timeline to make sure that you're still on the same page.