OP, YANBU. There are hundreds of threads on here about how parents should help their DC and not charge them rent, or help them through Uni, or with a house deposit.
Your parents are fortunate to be in those circumstance. I wouldn't ask them for a sum of money, but explain your situation and ask if there is any way that they could help you. They could lend/give you the money (for mortgage purposes it would have to be a gift though). They could buy a share of the house. There are several options, but they could have other implications, so they should take legal advice.
When my XH left, I was several K short of the amount that I needed to re-mortgage to stay here. I asked my brother to come on the mortgage with me, or if they could borrow and lend it to me, (I could afford it as had been paying far more on previous mortgage), but him and my parents chose to gift me the money (he inherits the family business, so on paper will end up with far more than me and I have understood this since childhood).
I WFH and by them helping me to keep my property, it meant that I also kept my office, and was also one less disruption for DD who was only 4 when her father walked out. It enabled me to build my business up and to be able to provide for me and DD.
Any decent parent, who has the funds, would probably want to help their DC out.
As you say their wills can reflect whatever they do, so that your brother still gets an equal share.
My parents took the view that it was better to help me when I really needed it, rather than leave me money when they go, which would be several years down the line.