dont quite agree there. Let's name an example, we all have those life long friends that you met in highschool or even elementary (just when you were young) and decades later you guys are COMPLETELY different, but these friends are so engrained in your life and you guys just get the way eachother thinks and have reels of inside jokes, that you can't help but just GET them (and their fucked-upness , sorry for making up a word) even though you both don't share any values and such anymore? We all have friends like that. Later around College - later twenties, you make the friends you choose, which is vastly different, they are usually more compatible to your personality, growth, and maturity - and so on and so forth.. But your "growing up friends" those you are usually stuck with for life.
Carrie seems to be someone that's engrained in Lydias life, and viceversa, people with that much history can't pick to walk away, it's like yes we're different but we're still family. I don't believe Lydia has become this way because of Carrie, more like inspite of. <3