As long as the cash gifted to them and properly documented on the gift tax form, I don't see a problem with this. Of course, for you to get the money back (i.e., not as inheritance), they'd need to gift it back to you. You didn't mention other folks in their wills (e.g., your siblings), but if wills are being constantly redrawn in concert with your gifts, then it is in the same legal territory as the world's oldest profession: when does being a "sugar daddy" turn into being a "john"? Now, if you're the only inheritees, then there would not need to be such redrawing, and you would get your money back through inheritance.
Something to think about is that they having cash could cause issues with means-tested benefits and lawsuit exposure. I'm a cheapskate that loves to dodge taxes, but I wouldn't do it.