If it's something you suspect is about to happen, I'm going to make the leap that you already know the nature of the kinds of things they are going to try to get you to do. In which case: I would suggest simply deciding ahead of time what you're willing to do and what you're not. Use pen and paper to work out for yourself what the absolutely maximum is that you will do, and then make it a firm offer when the time comes, when they start asking or hinting or however that occurs. "What I can do for you is xxx but that's it."

I know it's complicated when it's someone you're close to. Who knows how to pull at your heartstrings, and other strings. In the end though, you're not doing them or your relationship any favors by letting them push you around, though they may not see it that way in the moment. Whatever the cause is, whatever their diagnosis could really be if there is one to be made, for you to be saddled with the experience of feeling manipulated when already struggling with issues of depression is a lot to deal with. Be good to yourself.