Let me play the devil's advocate for a moment:
I don't know who put down the down payment on the property, but if it was brother or if there was no down payment needed for some reason, that impacts my argument.
Brother is making the mortgage payment which includes, interest, principal and escrow. (property taxes and homeowner's insurance). I doubt if he has renter's insurance on the property although he might. He did the renovations, and he apparently is taking care of maintenance.
I can just about guarantee that sister has not been including a Schedule E for a rental property on her tax return for the last 7 years. However, equity in the property has been building up due to the principal portion of the mortgage payment as well as appreciation. Therefore, if the argument prevailed that brother is a tenant, and sister a landlord, if she were to sell the property or the sale was forced through bankruptcy, she would be selling it as a landlord. Since a landlord must recapture depreciation, allowed or allowable, sister would have a very hefty tax bite upon the sale of the house.
If sister was selling the house on contract, then she has a different tax problem. She should have been reporting the interest and principal paid to her on her taxes on an annual basis. She would not have a depreciation recapture problem and she would not have a problem if it was selling at below today's market because the price would have been established 7 years ago, not today.
In either scenario though, sister has a tax problem. However, if it weren't for the laws in so many states that require real estate transactions to be in writing to be enforceable, the actions of the parties would clearly show this as a contract sale. Sister has certainly profited (on paper) by brother's payments and maintenance of the property based on the gained equity.
25k is a significant amount of debt but not an insurmountable amount of debt. No matter what the scenario is, if sister cannot service her debt brother's continued use of the house and possible future ownership (or current ownership in his eyes) is at risk.
So, if these were my family members or clients, I would be suggesting that the two of them sit down and have a serious discussion about how the both of them screwed things up 7 years ago by not doing things right, and how they can fix it going forward so that the problems are mitigated.
Some ideas:
Sister gets a HELOC to pay her debt (assuming that she can qualify for one with her present financial circumstances).
Brother and sister do a contract now, based on current market value, and give brother credit as a downpayment/sweat equity for the money he has put towards the principal and maintenance/renovation and his labor. Or, perhaps partial credit depending on other details unknown to us or because the situation has gone on many years longer than sister anticipated. Address the home equity loan in the contract with a stipulation that sister must pay it off before final settlement or during final settlement.
Another idea:
Same sit down discussion about reality, but they agree that sister sells the house now, gives him some money for his contributions to the equity gained in the property, and takes what she needs for taxes and debt and they go their separate ways financially. Brother won't like it, but he will like it even less if he loses out completely because sister is forced into bankruptcy (if the situation is dire enough for that possibility).
Both of them should talk to attorneys before proceeding in any direction on this.
Sister will have to understand that going forward she has to address the installment payments on her taxes and pay any tax due.