My Sister Is Attempting To Oust Me Out Of A Shared Property So Her Boyfriend Can Move In. What Can I Do To Prevent It/protect My Financial Interests?

location: california, usa

Background: My mother passed away in late 2021, her home was put into a trust where me (22f), my brother (26m) and my sister (32f) are the beneficiaries and our grandmother is the executor of her will.

In 2023, after the house was rented for a while my family worked with a lawyer and made a lease for my brother and sister to move in (and my nephew-sisters kid). They had a lease up until the end of 2025. Included in the lease was payments and rules that stated that no one else would be living in the house until I came back from college. I just moved back in last week but according to my grandmother, she used thousands of my money to pay and some of her own when my siblings did not pay (I gave her access and generally had no issue with it as ik she will pay me back) + (my sister who is supposed to pay majority didnt pay for 3 months and my brother hasn’t paid his share in 6).

But, my sister got pregnant while she was here (baby is a dec born 1yo as of today) and in my absence has *Ive realized started trying to move her bf in here, keep me from being comfortable in the home, and her (and brother) have decided to keep living here refusing to sign a new lease.

During a recent conversation my sister said several alarming things.

1. Her bf gets mail here

2. Im the only one with issues with the living arrangement (my issue is that he comes in and out of the house is a stranger to me, treats our backyard like a car shop, brings over strangers, regardless of whether or not my sister is here, and he NEVER takes over childcare, he has never watched the baby alone ever)

3. According to some bankruptcy lawyer if she files we will lose the house (she did not clarify if that meant forced to sell or alt)