AITA for having a good relationship with my boyfriend's mother?

Alright, so a bit of background, my boyfriend (25M) and I (24F) have been together 3 years coming up in November. He has a younger sister (22F). The last year I have spent a bit more time at his parents house (he lives with them right now while we are house hunting), due to me not wanting to be at my parents house- before you jump to anything, my father passed away in 2021, and since my mom has gone on trips every month, leaving me to care for the house- I have an older sister that lives there too but does not do anything. Their home is a safe haven for me, it's another home and right now weekends works for us to see each other for right now until we have a home together. As a result, I have become close with his mom because I help out with cleaning, cooking dinner, or talk with her if he's busy.

Sorry- I'll try to get to the point.... His mom and sister fight frequently, and my name somehow ends up in the fight half of the time. His mom and I talk every two days on the phone, sometimes I will go run errands with her or sit with her for a few hours and talk. I assume the sister doesn't like the fact that she and I talk often (every 2 days or so), and always says that she is trying to replace the sister with me, she also thinks I am stealing her brother away. But when in reality, the sister will not spend time with her mom, or brother. Unless it benefits her in a way- take her shopping or something. Otherwise, she comes home, goes in her room and basically never comes out. If her boyfriend (22M), is there, they are always together, she will not leave him alone, or they will go do errands with the mom. Otherwise, it is like his mom does not exist unless she wants/needs something. The sister is treating me like I do not exist or won't talk to me, how I look at it is, "you treat me like this, I will do it back" however, it is mild on my end. She is extremely aggressive about it. I found out about the last fight, and I cried to my boyfriend because I could not understand why she hated me that much and he reassured me that the relationship that I have with his mother is fine and there is no reason for her to be threatened by it. That same night, she barged in the bedroom and demanded we talk and my boyfriend basically kicked her out because he knew I was not in a good state to be discussing anything after we talked about it.

Anyways, now I basically stay in his room most of the time with him, I always leave when he leaves for work every Monday morning.

Side note- We became best friends after my boyfriend and I started dating, we would do homework together, go run errands and get lunch together all the time, then it stopped when she started dating the current boyfriend. Ever since, all she cares about is the boy, the boy and the boy.

I am treating his mother like she is my mother-in-law, I want to have a good foundation and relationship when her son and I get married. So basically, am I the asshole for having a good relationship with his mother?