Oh shit OP!
I'm so, so, sorry you've had all this going on...
.
You've had such major losses with you bro and baby and them Ptsd on top of it all.
I disagree with the idea of 'baby brain' that others have mentioned ...
I suspect its the other way around... What you're feeling is the insecurity of a man that you most likely wouldn't be with if you hadn't had kids with him?
I think you're making excuses for his poor behaviour.. You really don't seem that 'into' him.
Re Names I'm pretty sure you can call your babies whatever you want... Many of my friends their kids took their name for example dad surname was Green and sje names the kids Brown (her surname).
Also of you really want his name? I don't suspect you do? It's more to do with your wanting to feel a family (understandable!) ... There's nothing stopping you calling yourself by his surname. I do this sometimes with my partner - sometimes when we're travelling etc...
His family sound appalling.... To me that would be an extra reason to not continue to be with him.
There are lots of useful stuff legally about being married..
I don't think thats your problem... I think it's an unsupportive arse of a boyfriend... (of he can't support you in these circumstances... When exactly CAN he support you.
Please don't feel there is something 'wrong' in you. What you're feeling is completely normal given all the things that thane happened.
Please give yourself time and space to heal.
Stuff his family. Stuff their idea of their wedding...
YOU are worth so much more than this