How to solve complicated financing of family members?

I believe if the loan is also handled through Wüstenrot, you don’t have good chances there. I started from my understanding that loan agreements often contain a clause (maybe not that often? I thought it was common, but I could be wrong) that a prepayment penalty is due in case of early repayment. That at least leaves the option open to repay early.

But I don’t know such configurations at all. More common are these building savings contracts, where at some point the bank loan from any bank is replaced by a building savings contract + building savings loan that is ready for allocation.
In your situation, there is basically a separate loan agreement (but also with Wüstenrot – a building society?!), and a building savings contract that is ready for allocation, but you do not accept the allocation; there is basically just money sitting there waiting to be withdrawn when the other loan with Wüstenrot is "low enough"?

I would simply go to the notary, buy the house/land with the condition that it must be delivered free of encumbrances. Then the notary automatically contacts the bank and requests the deletion documents (with prepayment penalty). Afterwards, see what happens. It is not the buyer’s (= you) problem, but your sister’s. Let’s see what the bank says. Stress means that the bank is behaving difficult.

I would simply go to the notary, buy the house/property with the condition that it must be delivered free of encumbrances. Then the notary automatically contacts the bank and requests the deletion documents (with prepayment penalty). Afterwards, see what happens. It is not the buyer's (= your) problem, but your sister's. Let's see what the bank says. Stress means that the bank is being difficult.

I did not think it would be "that easy". I am very grateful for your response.

Contacted the agency yet? You also need a refinancing ... maybe something can be done

is the financing for the house that is to be demolished or as additional security for another house. If the income is right, the bank sometimes waives collateral. The financing is with Wüstenrot Bank, the building savings contract with Wüstenrot Bauspar AG

Have you contacted the agency? You also need refinancing ... maybe something is possible

Our agency from back then no longer exists. Unfortunately, the next one is farther away. Unfortunately, we were disappointed often enough by phone, so collaboration will be ruled out.