Is it not right for asking my sister and BIL to pay me rent?

Okay so short backstory: My parents bought a house to raise their family in and decided to both officially keep half of it in their name after their divorce.

They paid it off together and when my mom died her half got split up between my sister and me, so a quarter of the house is officially in each of our names. The other half still belongs to my dad.

I (25f) moved out 6 years ago and my sister (27f) has stayed in the house with her husband (29m) and their son (8m).

When I first moved out we agreed that they should pay me rent, since it would be better for me if we rented the house to someone else to get some extra income. Until now we really haven’t followed through with it, because I had a full-time job, could afford my life by myself and they both were still in school.

Since then a couple of things have changed: I quit my job and started University last year. But with my rent, bills, food, books for Uni and the semester fee I have to pay, I still have many expenses.

So besides going to Uni full time, I now have 2 jobs to pay for everything. Until a little earlier this year everything was fine, but since everything has gotten more expensive I really can’t afford my life anymore. But with work and studying, I’m already at around 60 hours a week, so I just don’t have the time (or energy) to get another job.

I remembered the agreement a while ago and have talked to my sister about it, who refused to pay stating that they don’t have enough money either. Both of them have full-time jobs and because they live on our owned property they don’t have to pay rent, just the costs that come with a house.

I’m asking for about half of what I could get if we rented the house to someone else and got a quarter of that amount, so I don’t think I’m being unreasonable. It still would be better for me if they moved out and we got tenants.

My dad got involved and said that they really don’t have that much money and can barely save anything every month, which really upset me, because I had to dip into my savings each month to cover rent and my bills. My best friend doesn’t think I’m wrong here, because it simply is what we agreed on.

how?