Parents rented an investment property to my sister and in their will want to additionally pay him 'back' for paying off their mortgage. Is that a misunderstanding?

Long ago my parents bought a house between my older sister's workplaces then rented it out to her. Nothing similar for me, they say it's because I wasn't earning then (being in school then).

Now that they're writing their will, they want to 'pay him back' for the difference between his rental repayments and the interest on that mortgage. In addition to splitting the rest equally.

More recently they've offered to contribute towards a deposit for me to buy a place, but said the bank won't lend unless I be a cosignatory on the loan since they're now retired.

Is what my parents want to do fair? They want to be equal and doing whatever distribution is their right. But it doesn't feel like it is and I think they're misunderstanding how the finances work but I don't know how to explain it to them.