Cutting off my soon to be SIL

So I'm not really fond of one of my SIL. She makes everything about her and is really mean sometimes. Well she let me know that my boyfriend helps pay some of her bills and gives her money. We are getting married next year and I think when we do I want to suggest that he cut her off of his money. We will have two kids by then and she is a grown ass woman with a full boyfriend and a good job. She made it clear a few months back that she doesn't want me coming in between her and the money my BF gives her to help out. I feel like a bitch but it has to stop. She's basically mooching off him. She needs to learn to budget or make her man step up more on bills. But I would love to get him to stop giving her money for her bills every month. I think that's crazy as hell. Am I wrong? Do you think that would be mean of me?