Unreasonable MIL

My hubby and I bought a big enough house to accommodate my parents & sis in law. Total of 4 bedrooms - 3 taken by my in laws and 1 for ourselves & 9mo son. In order to help my pil retire comfortably in the next 2 years, we decided to let them stay together with us and rent their own house out as a mean to collect passive income for retirement. Fast forward 1 year later, things got very uncomfortable to continue staying together. My mil brings up money matters to argue with us every other day. Back ground info - She's working full time at home with 3k+ income, has an allowance of $500 from my hubby, rental income of 2.3k (to spilt with my fil) but v unhappy to be spending the money to buy groceries and cook for our family of 6 (approx. 6 times a week for dinner only). We also hired a helper to look after the baby to relieve my mil from child caring duties while both of us continues to work. Me and my hubby paid for the SP bills, conservancy fees, Internet bill and buy rice, cooking oil, washing detergents, tissue paper for the household. I don't mind losing out a little in terms of money but she has been giving me a lot of unhappiness. I cannot comment on how she oversees my helper and cares for my son. In short, I'm supposed to close 1 eye in everything that she does. The best is not to make any comments at all. Our differences are increasing gradually. This got magnified further with an incident that happened on sunday. My friend surprised me with a upsized cup of bubble tea and I met her at the mall to collect it. When I returned home, my sil was very angry that I didn't get one for her (I will always get an additional cup for her whenever I buy one for myself) and slammed her bedroom door (I didn't know this until my mil texted my hubby the next day to tell him about this). My mil said that it's my fault that I didn't even ask to share some with my sil and if I were to only get one cup, I must go up to my room and drink discreetly. What will you do if this happens to u?