I dunno, the term 'LGBT lobby' usually sends up a bit of a red flag for me.
As much as I don't like to tell other people their business, in the (approximate) words of Dan Savage, the best or only leverage you can have over your parents is your presence. If her intolerance is going to characterize your relationship with her going forward, then you're not obligated to continue to see her on the basis of familial ties alone.
I hope that kid's alright, though. If they turn out to be LGBT that's gonna be a rough ride for them...