I think she never recovered from her initial decisions as a young adult and then all the other unfortunate things that happened with her. This coupled with mental health issues and the years just went by. 
First, when she decided not to complete her studies, her parents sent her to a finishing school abroad so that she could learn societal skills and be a good homemaker. She came back early because she wanted to marry her boyfriend. Remember reading all this in an old interview where she goes into details about how she tried to make her abusive marriage work. 
Then her daughter asking her to let her go and live with her dad, the other failed relationships, obesity, depression, and when she was finally working as a producer (krazzy 4) , she got diagnosed with cancer. 
Not trying to say she that she is completely faultless. Sure, she has to be accountable for a lot of her choices too. Just saying that I feel that a few times she did try, but something else happened to pull her back. She lost her young years trying to make her marriage work, then her middle years when she was trying to start a career , she got cancer. There were also other health and mental issues.
A few times when she could have been stronger and more sensible, she ended up taking stupid decisions. 
If she is touching 50 and has never held a proper job so far and never completed her studies, then it is sort of late for her to "become independent" now.
Her parents aren't exactly locking her up at home. If she wants to leave, she can. But obviously, she can't. I think her parents don't really have any delusions that at her age, Sunaina will magically start earning for herself. I wouldn't be surprised if they have already set aside some assets, money or inheritance for her to live off the rest of her life. But they won't just hand out everything to her if she can't take care of herself physically, mentally and emotionally. Having a drink just after coming out of rehab doesn't exactly inspire confidence. She is speaking like a typical addict who can relapse anytime when she says "what's the big deal if i have a drink or two?". You don't hand over money to addicts.
If she wants the money, she has to first earn their trust. Then, their confidence - show them that she can manage the money. That she can make it last for years to come instead of blowing it all up.
If all the reports of her suffering from severe mental health issues are true, then I don't blame her parents even if they end up doing something drastic later - like asking some sort of adult guardianship rights from courts (I don't know the law so not sure if that's a thing) to restrict her. Because then it would be a case of trying to keep her alive. 
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