True Brendan, once the OP takes on board the advice here and the risks.
Your earlier message outlining the options is the first thing to think through.
The most suitable option should be chosen only after the OP and her husband have discussed the risks to their own financial situation if her sisters marriage breaks up or they can't pay back the money for whatever reason.
Then, insist on a legal agreement being written up and agreed by all 4 people involved with all potential exit options agreed and if necessary, all 4 should have a will which states what happens if they die.
Family and borrowing money is risky and she will always be your sister .
If nothing goes wrong, it could all be fine, but not discussing the risks in advance increases the seriousness of any row that might happen.