They're about to fall out because the OP's sister wants to go back on their written agreement.
Think of it like this.
OP and Sis: Let's buy a house and live in it together! We own it equally.
Four years later:
OP: My circumstances have changed. I plan to live with my boyfriend. Can we sell the house and split our money?
Sis: No. I wouldn't be able to afford to buy a house with just my share of the proceeds and living here is cheaper for me than renting.
OP: OK, well I can't afford to pay for my half of the mortgage repayments as well as renting somewhere else. I won't charge you rent on my half of the house but you need to cover the mortgage on your own. Can you do that?
Sis: Yes. It'll be tight but I'll manage.
OP: You could get a lodger to make it more affordable for you.
Sis: No, I don't want to.
OP: OK well whatever you think is best.
Two years later, house is sold, solicitor distributes proceeds of sale equally in accordance with the sisters' equal legal ownership of the property.
Sis: Now I'm going to sue you for two years of mortgage repayments even though we had a written agreement that I would cover these on my own.
Where's the middle ground here?
The OP bungs her sister some more money even though her sister is legally in the wrong and is also the reason why the OP's money has been tied up in a property she didn't live in for two years?