Heating children's room, bedroom, and bathroom

"the" my grandma and grandpa. The house belongs 50% to my mother and my uncle wants/has to sell it because of the running costs.

Then I would wait with the renovation until you are registered in the land register. A renovated or partly renovated house is suddenly worth much more than an unrenovated one...

Claudia1985, some questions arise:
- Who owns the house and in what shares? (50% mother, 50% uncle)?
- Is there currently an inheritance case ongoing or is it in the past and settled?
- Is the house inhabited? By whom?
- Why are you taking care of radiators?
- What condition is the house in?
- What condition should it be brought into?
- What should it cost? Who should pay for it?
- How should the house be used in the future?
- ...

Of course, you are not supposed/must not give answers here. It just seems to me that you are not very clear about the situation yourself. If that is the case, then a short pause and sorting out your thoughts (and money) could perhaps save you from some trouble. You should try to keep emotions out.

The house is 50/50, my mother transfers her part of the house to us and we pay off my uncle. My grandmother has a lifelong usufruct right (she is 88) but lives with my uncle. We pay for it. The house has been empty for about 7-8 years. The house is to be inhabited by us (husband + children) in the future. The roof is new, newly connected to the sewage system, the courtyard is newly paved, and it has new electrical wiring. I am concerned about the radiators because I cannot estimate what kind of ones such an old house needs, or how it is with the clay walls. There are some on the lower floor.