Listen, my family is a lot like your gf's. I love my mom, but I have to keep her at arm's distance. Yes, she raised me and got me tutors and toys and loved me. But she was also a narcissistic alcoholic who emotionally, physically, and sexually abused me, my dad, and my stepdad to varying degrees.
She's sick now, which compounds things. People see this frail mom of mine and know I had a privileged upbringing she provided, but they didn't see when she "simulated a rape scenario" when I was 13 to "toughen me up and make me fight back" (not an actual rape but very traumatizing). They didn't see her neglect and abandon our family pets that SHE bought. They didn't see her drink entire bottles of vodka and then keep me awake until 2am on a school night when I was 11 to tell me stories about her sexual escapades and abusive ex husband. They didn't see her, more recently, tell me I was worthless and useless and if she killed herself it would be my fault. They didn't see how she openly admitted to trying to use my former stepdad for his money, and how she beat him black and blue against his consent while he was tied up in a BDSM situation.
We talk on the phone once in a while and touch base, and of course she brings up health problems and asks for favors every time. And it hurts me to say no, but every single time I have, it has resulted in her calling cops on me(!!), being emotionally vitriolic, and making me feel like an empty shell of a human being. I am afraid to ever become like her.
My ex-stepdad and his current wife are my family now, and I love and respect them, as they treat me the same. I love to visit my extended family. My grandpa may have been an adulterer and he's a little obnoxious, but I would never write him off just for that. You have no idea what she's really experienced. Trust her to make her own decisions about her family, and if she is otherwise compassionate you have to understand there are probably reasons she wants limited contact with them.
Congrats on having a nice family. Not everyone is that lucky.