My sister [35 F] is a servant and unhappy in her arranged marriage [35M] and I [33F] don't know how to help

Disclaimer: I personally don't want an arranged marriage, so if you don't think it's a good idea, you don't have to convince me. I don't judge people who have one, though. About half my relatives and a few of my Indian friends have one. It's just my personal preference.
That being said, thanks in advance for any advice.
My sister, raised in America since age 2, got an arranged marriage at age 23 to a doctor from India. He moved here after the marriage. Pretty soon after, her in-laws moved into her 2 bedroom apartment. This is the tradition, for the husband's parents to move in after marriage, but a lot of Indians don't do this. Some do, though. Maybe half and half.
She worked while he was in residency and while she had two boys, and now she is a stay at home mom. While she worked and had kids, she still had to make full Indian dinner every night and chai in the morning. She did all chores by herself.
She is basically a servant in her own house. She cooks and cleans for her family of 5. Whenever she makes a special dinner, like Punjabi food or Italian, she also has to cook for her sister-in-law's family of 4.
Her mother in law doesn't lift a finger to help. She watches TV, naps or talks on the phone all day. She watches the kids, but doesn't make them meals. She leaves that to my sister. She doesn't take the kids to the park or on walks. She doesn't supervise their homework.
If my sister wants to go out to have lunch with her husband, she has has to lie about it. She says she has a "doctor's appt". I know this because my BIL told me. He thinks it's funny that they have to lie. This is because the mother in law will want to come. She doesn't have friends, either. Even if she did, I don't know how she would go out when she has so much work to do at home. Her in-laws come along on vacations.
I told her that now that BIL is a full doctor, they can afford to get a maid to come by once a week or month. She said that would never work.
Important info: In Indian culture, the husband's mom is basically a queen and the wife is supposed to serve her and her father in law, but these days and even in my mother's generation, this is often no longer done. I don't know a single other Indian family where the MIL abuses the old traditions in this way and refuses to lift a finger.
For example, my grandma lived with us, and she would start dinner when my mom was at work. She would eat leftovers or make lunch when my mom was at work or busy. She would wash dishes even when my mom begged her to stop and sit down. My sister has to make a new lunch every day and clean by herself. My grandma also took us to the park, walks, and supervised homework or playtime.
What can I do to help? Anything? I understand that her husband should help my sister and tell his parents that my sister needs help, but I don't think he will, since he's not dumb, sees it, and hasn't talked to his mother yet. He is a huge Mama's boy.
This is also affecting my relationship with her. She is not nice to me and insults me often, even though I treat her and her family with respect and am hands on with her kids. I shower them with love and attention. I take them on outings to give her a break when I'm in town, 3-4 times a year. When I'm in town, I watch them while she leaves the house to spend time by herself. I think she is unhappy and resents the freedom I have in my life.
I also think all the work takes away from her time with her boys. She doesn't have time to play with them and I don't think she helps them with homework as much as she could, if she had the time.
TLDR: my sister is a maid and servant and seems unhappy in her marriage. Can I do anything to help?


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