<human>: Can you condense the text into a few key points?

Earlier tonight, I made a joke about my girlfriend being not as good enough as her older sister. I seriously didn't mean it, but she got very flustered and upset. 

I didn't know it was off-limits. I apologized to her, but can still tell that she's not over what I said. 

The thing is my girlfriend and I are both sarcastic people and like teasing each other. It usually is fun and funny (we're both witty people! :-)) but sometimes it goes way too far. For example when she called something I proposed stupid, it was not cool with me because I found it degrading. 

We are terrible at communicating about feelings. It seems like we either have to a) not tease each other at all and tiptoe around feelings or b) have this problem. 
   
I know theoretically what should happen is we should define the boundaries of what is okay to joke and rib each other about and what's not, but we're both unwilling to have this conversation. Me, because I feel so vulnerable telling her what I'm sensitive about; it seems like a weakness and I don't want her to see that part of me.  

We've been dating for 5 months btw.
<bot>: My girlfriend [29/F] and I [27/F] go too far with teasing and cross the line into cruel remarks but we don't know how to communicate what's off-limits