I'd want to be a partner - 50/50 ownership. I personally would not give somebody the money to buy a property, and then let them be the only name on the deed with the possibility they would blow me off and then keep the property.
If your relative isn't okay with that, then I don't think you should give them a dime. Because you have to wonder why?
You can love someone and also be wise at the same time. You don't do anyone any favors in life if you take up their life consequences for them. In the real world, you don't get to buy properties without any skin in the game. So, if you give them your skin, without putting them in a position to lose any themselves if they don't act responsibly, and then only you suffer the consequences - you haven't allowed them to learn any real life consequences for their behavior. That's not doing them any favors, and is definitely not a wise thing to do yourself.
If you're not comfortable with that, then just give them the money and call it a gift. And then, let them take the consequences as far as having to pay taxes on it :-)