Shares house/mortgage ownership dilemma

Hiya ladies,

My sister has reached out to me for a chat about her situation with buying a house and I personally feel as though she is taking a risk with her life savings. As it’s a bit personal I was hoping you ladies might be able to shed a neutral non-biased view on the situation 😊.

My sister and her long-term boyfriend have just had an offer accepted on a house, they are both first time buyers. My sister has always been the big earner and saver, whilst her boyfriend is a much lower earner and has no savings. They have always kept their finances separate for various reasons.

The house they are buying is low value due to the work that’s needed on it and my sister is contributing with a 60% deposit. The mortgage they are applying for is 40% which they plan to split between them. So, financially my sister is putting in 80% and her boyfriend 20%, which is how I have suggested they split the house ownership - SIMPLE.

However, my sister has an extra 15k to do the house up (requires extensive work), whilst her boyfriend has nothing to contribute. The argument is that she should have extra ownership because of the investment she’s putting in to doing the house up, whilst her boyfriend is against this idea. Apparently, he would prefer to split the house ownership 50/50 and he has given his word to allow my sister to have whatever she put in returned to her whilst all profits from selling the house later on is split 50% - doesn’t seem fair or a guarantee really. He argues that he’ll probably get 10k worth of inheritance at some point in the future when one of his relatives passes, however those funds aren’t available now whilst this person is still fit and healthy (a depressing argument point)....

My mother has mentioned that he might be emotionally blackmailing her and my sister reached out to me today to talk about the situation. She appears to be being chipped away by her boyfriends stubbornness over it. Personally, I feel that as they are not married, have no kids and have always had separate finances that they should split the ownership according to the amount they are each putting in. Any additional investment/profits from doing up the house they can split at a later date according to however much they put in from my sisters savings or his inheritance (if he gets it) etc... I do feel as though she’s getting a rough deal with her boyfriend, my mum in particular is worried about how her boyfriend is approaching the situation - I live quite far away so Im not actively aware of how he’s been talking to her, but I am quite worried!