not legal advice or anything but I have been in a similar situation to you, I have lent my parents thousands over the years. It is hard to say no as they are family and there is a lot more to it then "just stop, lol"

However, you need to realise they are adults making adult decisions, they will continue to go to you because you are letting them. No one will stop doing something that benefits themselves when there is no consequence to it. So while it is hard to just stick to your guns and say stop, it really is that easy, they might pressure you a little at first or be 'disappointed' in you, they aren't really, they just don't want to change their normal.