Guardianship Question?

I live in Michigan and I have legal guardianship over my mentally disabled sister and I make sure her bills are paid and her needs are met each month. Here's my concern. She pays 100% of the monthly rent which is $515 a month and he pays most of the electric bill. In my opinion I think my sisters boyfriend should be paying half of the rent and half of the electric bill and then I would pick up my sisters other half of the electric bill.

His excuse is that he pays the electric bill and he shouldn't have to pay rent. Then his next excuse is that the disability money that comes in for my sister each month is her money and she should decide. In my opinion, Yes, the monthly disability money is hers. But, There's a reason why the court appoints guardians is to stop people from taking advantage of vulnerable people that doesn't know any better and I don't think it's my sister's responsibility to carry her boyfriends burdens for him. He is able bodied, He can work. He seems to have money when he wants alcohol and cigarettes. But when it comes to getting down to business and taking care of responsibility. I feel like I'm stuck at a crossroads on what to do in this situation.