If grandma gifts the house to the evil couple (or whatever), they would have to cover any potential care costs for grandma within 10 years. However, this only applies if grandma has no other income. This is apparently called induced impoverishment.
How it would be with a will in which others are entered as heirs, I don’t know. Probably you would have to pay a balancing amount, from the one who was gifted to those who are to be the heirs.
This can be a huge mistake for the recipient. For example, if you assume that 5 years of care costs have to be covered and then an inheritance balancing payment has to be made, probably nothing is left of the value of the house or you even have to pay on top.
Selling is no longer that easy to do for 1€. The state has mostly closed the loopholes. The market value (I believe) is taken into account and monthly payments have to be factored in with the average life expectancy of the respective year group.
At least that’s how it was.
I also don’t know why everyone always gets so worked up when it comes to inheritance and the like. The sooner you solve it together, the better it is for everyone involved.
All in all, I’m not an expert, so it’s probably not as easy for the evil couple as one might think.
Even if grandma gifts the house to the couple and dies x years later, the heirs have a right to the registered value. I strongly doubt that this simply dissolves with the gift.
Especially if the inheritance comes from grandpa who is already deceased...
Only a notary or lawyer can answer such things. The situation quickly becomes very complex and hardly understandable for laypeople.