Your sister wouldn't have been able to secure further borrowing against the property without your consent if you are named on the title of the property. I am presuming when you bought the property your sister and you used solicitors who put you on the title as tenants in common.
If your sister has secured further borrowing against her name, but NOT the property, then you have nothing to worry about.
Charges on the property will rank in order of the mortgage lender first (Halifax or whomever) and then your charge securing the £43,000 you lent to her (again, I am presuming that this was placed on the title in the form of a second charge - any solicitor worth their salt will have ensured you protected your money like this).
If she has taken an unsecured loan out, that's her and her ex-husband's problem. It won't take a chunk of equity out of the house.
When the solicitor is working on the sale of the house he will contact the first mortgage lender for a redemption figure (this may include the unsecured loan, along with early redemption charges etc) and he will also contact you - or it may be clear on the title how much your charge is worth, perhaps it's expressed as a percentage?? Once the solicitor has ensured he has enough money from the sale proceeds to be able to discharge the loans on the title of the property he will then go ahead with the sale. It is a legal requirement for the solicitor to have ensured this in the first instance. If there isn't enough equity - ie if the house has £130,000 worth of loans against its title and is being sold for just £120,000, then the solicitor has to put things on stay until the seller forwards money to him to cover the shortfall.
From what you've said, you own 1/3 of the property so there's little doubt that you are named on the title. You are probably named as tenants in common with the percentage split 66.6% your sis and 33.3% you. If this is the case, and there's nothing else such as a charge entered on the title or a specific delcaration of trust mentioning the actual amount involved then it could be that you're just down as having a percentage share.
Really, we need some more info on what exactly happened at the time of purchase.
Also, your sis's solicitor will need your sig on the transfer deed and the contract for sale so surely they've been in touch with you about this anyway? If not, they will be and you can just ask them.