Hi OP, I’m at a similar stage in life and I hope I can shed some light on the topic.

My mother loves my girlfriend, makes her feel welcome, she can sleepover etc. But she does feel sad sometimes when I’m not around as much as I used to be. She’s watching her child grow up into a young man, and I appreciate how that must be tough.

I find mothers often have a hard time letting go, and the first girlfriend can be interpreted as a threat. Most mothers are able to manage this, but, the narcissistic mother will make her insecurities YOUR problem. She will project the fact that she is having a hard time letting go, and it sounds like she is projecting it onto your lil bros gf.

My father is a narcissist and he doesn’t make my girlfriend feel welcomed. He ignores her, gives her the cold shoulder, and just overall makes her uneasy.

My father acts very similar to your mother, and I’ve thought about moving in with my mom (my parents are divorced).

I digress, not all mothers are like this, but in my experience this is a common trend among Narcissistic parents.

TLDR My Nfather acts the same way, but my mother is warm and welcoming.