Hire purchase within the family (with ownership share?)

This probably won't work because the house still somehow needs to change its owner. In the case of a gift, at least 2/3 of the value would be subject to gift tax.

I don't mean a gift, but a private loan. You could also do it in installments. So that you don't take over the whole house at once, but only 1/3 or 2/3. I would really ask a notary or lawyer whether there are simpler arrangements than rent-to-own, etc.

At the moment, the community of heirs could still claim it for tax purposes as far as I understand, although we pay it, but if the whole thing was considered a hobby due to a lease purchase without additional income from the rent, no one could deduct it anymore – that seemed to me so far to be the only reasonably manageable financial disadvantage of this construction.

Since you currently pay no rent, the owners cannot deduct anything on their taxes, neither property tax nor renovation costs. No real estate transfer tax will have been incurred, as it is an inheritance, and the property tax is probably only 200-300 €/year.

Since you are currently not paying rent, the owners cannot deduct anything on their taxes

Depending on how long one (the testator) has already owned the property, possibly the depreciation of 2% of the building's value, but only if rented out at at least 60% of the market rent.

I would also work here with a purchase on installment. The heirs (mother/aunt/uncle) would then receive a land charge, preferably in second rank, as the first rank is more favorable for a bank loan.

For tax purposes, it would probably be more advantageous if the community of heirs renovates and receives reasonable rental income from you. The rent and the depreciation of the renovation could be structured so that the whole reduces the equity tax for the community of heirs. However, this means significant effort for the community of heirs in terms of tax declaration and proof of renovation costs. For a later purchase, you could get a right of first refusal. I wouldn’t want to impose something like that on my parents.