Best way to safely lend 10k to my sister for a mortgage down-payment?

Context:

My sister's in a bit of a tight spot. Her and her partner have been renting the place they live in for a couple of years, they have two young kids. Owners have recently told them they want to retire and sell the place. They are giving my sister a chance to buy the house before they put it on the market, but they'd need to cough up a 20k down payment to purchase the house by the end of spring, otherwise the house goes on the market and they need to move. The sellers are offering a pretty good deal (from what I hear) so they really want to stay, but they had a pretty expensive year (big dentist bills and a relative's destination wedding) so they can't pay that amount out of pocket and are looking to borrow.

So, she asked me for some help. I work a pretty decent job but I don't have 20k lying around. I do have enough to give her half (10k), but I'd be taking from my RRSP. I've already done all the phone calls and they told me that due to the plan I have with my employer, there aren't any risks involved other than the withholding taxes and withdrawal fee. They gave me an estimate for how much the total would be with a withdrawal fee and withholding taxes. My sister is willing to pay back everything it costs me to lend her the money (including withdrawal fee and withholding taxes). She has always been good to me and assured me that they would pay me back next spring when they get their tax returns, and I believe her. They are first time home buyers so she told me she should get a pretty hefty tax return next year (not really too knowledgeable about that but I'll take her word for it).

Actual question:

I'm inclined to help her, but here's my question: I'm wondering how to give her the money while also protecting myself? I admit I'm pretty out of my element here- I've heard that with mortgages, there's a whole bunch of things that need to be declared, whether it's a gift or a loan, etc? I heard with mortgages, you need to declare it as a gift as opposed to a loan, but I definitely want my money back and I trust her to pay it, so I'm a bit confused. Should I talk to a lawyer and get some kind of a written contract specifying that the money will eventually be payed back to me?

Thanks to anyone who is able to give me some advice!