I am not so sure they are doing this to make money. They will have a mortgage and costs to pay related to the purchase of the property. Suggesting you move into a place they own tells me it is in order to improve the cost of rents you have already been paying. Thus, I don't see that they are taking advantage of you.

Now, I DO agree with the statement that they are your landlord and as such are the determiners of the particulars regarding your rent. However, this is not to say you are not in a position to negotiate. It sounds difficult but I would speak up about your concerns - and set limits and boundaries regarding your expectations and arrangements considering your sister. Discuss what the point of taking action will look like, the precautions to reaching it, and action to be taken if and once it has. Make sure that this is all in writing. This is family yes, but you should still draw up a contract by way of a lease agreement. Good luck.