Let me start by strongly recommending against.
I speak from personal experience after buying a house with family almost 20 years ago. It seemed like a generous offer at the time, and it did help us get our foot in the property market. However, from the beginning there was confusion about who was responsible for what. Who paid what if we wanted to remodel, and who gets what say in the design decisions. Who pays for repairs, taxes, insurance, and so on. Who decides when it's time to sell, for how much, and how are the proceeds divided.
In our situation the "gentleman's agreement" was that in laws would pay the down payment, we would pay all expenses, and split ownership 50/50. FIL insisted he only wanted to be paid back for the down, yet we weren't even on the deed. I was young and dumb when we set this up, and being new to the family was too "nice" and naive. At some point we realized that we were paying huge amounts yearly, but since were weren't on the deed we were essentially renters and were likely to get screwed (if nothing else, on taxes). So we had to push very hard to get recorded 50% on the deed.
Then, as my career started ramping up I got the sense that my FIL was seeing dollar signs. Didn't know it at the time, but he has a history of starting with good intentions which is later superseded by greed and/or desire to control. So ASAP I insisted on paying back the down in exchange for his quitclaim on the deed (e.g. 100% ownership). As suspected, he started talking about wanting his share of the appreciation even though that wasn't the original agreement. The compromise was to pay back the down along with interest.
This was all hugely stressful and unhealthy for our marriage and our relationship with my in-laws. It also meant we bought more house than we needed way too early, and it meant we ended up in a house we didn't like all that much (FIL wanted his say). Financially we would have been way better off just waiting and buying on our own when the timing was right.
All this happened many years ago. We recently sold the house in question and were extremely thankful to have complete ownership. Was clear from conversations with FIL that it would have been nearly impossible to sell if they were still on the deed. And with the recent run up in prices I think he would have demanded half of the appreciation even though we would have carried all the expenses.
So let me finish with this: If you decided to proceed then make sure everything is divided clearly and is in writing. Think about it this way - do you want to marry into a situation where your spouse has unclear ownership with something as major as a house? Think about 40 years down the road or whenever the parent pass, the mess of siblings demanding their inheritance, a slice of your house even though you've paid all the expenses and have done all the work. And make sure you have a clear exit plan, again in writing. And make sure ownership is recorded on the deed. And have a clear plan for how this is handled if you get married, perhaps with a prenup that phases out over time.
