Splitting Ownership Of A House With My Parents And Siblings

My mom and my 2 siblings bought a house (in Oregon) a few years ago when interest rates were record low. Back then they didn't think much about it specially so far in the future, I had been living on my own already so I wasn't involved at the start. Years have passed and my siblings want to move out on their own but my mom cannot afford to pay for the mortgage on her own. My siblings have offered to split the mortgage 4 ways (them 2 my mom and I) and split the ownership of the house as well. I'm open to this idea; my siblings get to move out, my mom can keep living in her house and me and my siblings get to invest on a house that already has increased in value.

How can i legally go around this? What sort of documents or procedures do I need to make sure the house is split equally without having to sell the house and let go of the current low interest rate the mortgage has? My mom has offered to get a notary and write some letter but I'm not sure how legal this is and if 30 years down the line me and my siblings are going to be arguing about this.