Me 21m and gf 20f. My gf’s mom has no morals.

My girlfriend and I have been together for 6 years. Anyways I always hear the saying, “When you marry or date a girl, you are also dating/marrying her mother”. With that being said, my gf’s mom has a bf that I’m not too sure how serious the relationship is? but she has openly told my gf that she has cheated on him before and has hung with other guys in the past, but just recently my gf told me her mom has now been seeing some new guy who also has a gf who is obviously actively cheating on his gf too. Also it’s hard to get this info out of my gf weird enough because she says she doesn’t want to give out her mom’s info to me and it’s none of my business. My gf is nothing like her mom and hasn’t gave me any reason to think otherwise, she’s a very sweet loyal girl and I’ve never had any problems about loyalty. But I’m just starting to think what does her mom feed to my gf when I’m not there? She knows her mom is morally incorrect. Another thing is her and her mom are best friends, she’s not a mom to her she’s a best friend. Anyways I need help on if I’m getting myself into a disaster and God forbid a preview of what I will experience if i continue down the road in this relationship.