I need help with first RE deal

[David Bellmont]

My wife and I have been approached by her parents about purchasing the house that she grew up in and that we live in currently. The property itself has an older two family building on the lot and then a newer duplex attached by a breezeway (the city would not let them split the lot into two separate properties and any new structure had to be built attached to the old building). Three of the units are rented to tenants, one unit is where my wife's sister lives, and my wife and I live in an unrecognized basement unit which is technically unrentable because it does not have a second egress point. My wife's parents ultimately want to split all of the units into condos so that they can keep one for my wife's sister and sell us the older building with a chance to buy the third unit in the future. His one sticking point is always having one unit for my wife's sister to live in. Is there a way to avoid splitting the property into condos while putting in the sale agreement that the sister will always be able to live in her one unit? Also her parents are interested in owner financing the deal, but they still owe $170,000. Would we be able to take out a loan to pay off what they owe, and then they finance the remaining balance. Can anyone provide any insight to how we could structure the financing through her parents/the owners? Any help would be greatly appreciated. Dave