For those on this thread who are not lawyers (and there are plenty of you, based on the crap "advice" you've been posting), here is the definitive legal position (we are, after all, on the "legal" part of this site):
2 years ago OP wanted the house selling. Sister didn't want it selling at that time, so she agreed, in writing, with OP that she would pay all the mortgage after OP moves out until the house is sold. She dragged her feet in selling, but given the agreement they had reached, OP didn't force the issue.
The agreement did not alter their beneficial interests in the property. If that was the intention it would/should have been made clear at the time and also recorded in writing.
2 years later the house is sold. OP meanwhile has been prevented, by this mortgage, from investing in another property with her partner. This is legally irrelevant anyway, as they had a specific agreement that the sister would have sole occupation and for that she would pay all the mortgage.
This is what Halsburys Laws has to say about occupation rent:
It has been judicially stated that a court of equity will order an inquiry and payment of an occupation rent in any case in which it is necessary to do equity between the parties that an occupation rent should be paid. This will generally be so where one co-owner has ousted the other, but the fact that there has not been an ouster or forceful exclusion is far from conclusive.
Occupation rent is probably the wrong way to analyse this anyway - it is a tool of the courts of equity whereby a notional rent is inferred. In OP's case there is no need to infer rent, as there is an actual agreement to that effect (without mentioning the word rent - that is irrelevant).
Different people have different views of this. Morally I think OP is firmly in the right. She had an agreement with her sister. Sister should morally and legally be held to that. Had the value of the property dropped the sister would not have offered to bear the reduction entirely from her share.