Is it feasible to buy a second house with a mortgage on the first house?

No idea how the lifelong right of residence is taken into account

Discounted rental income based on the remaining life expectancy of the aging partner (according to the mortality table) is considered as a value deduction. It should be manageable at that age.

Then it gets expensive if the relatives are not exactly parents / parents-in-law and the exemptions are correspondingly high. Notaries also do not like a "low" price. They set a higher one (they also want to earn accordingly), even if it is not actually paid.

Additional costs (real estate transfer tax, notary) and gift tax (depending on the degree of kinship) you should definitely have. But it would also be far too risky for me. They could live another 10 years and with your income, there is also little tax saving.

I roughly calculated earlier and I think it came out somewhere between 70 and 80K€ for the right of residence.
Gift tax exemptions for distant relatives would, I believe, be 20K€ per person. But if 2 people give gifts to 2 people, then you might get to 80K€. The rest of the gift would then be taxed at 30%.
But this is just dangerous half-knowledge.
Still, there is easily a thousand missing, which would have to be paid monthly additionally to Ratze.

I roughly calculated earlier and I think I ended up somewhere between €70,000 and €80,000 for the right of residence. The gift tax exemptions for distant relatives would probably be €20,000 per person. But if 2 people give gifts to 2 people, then you might get to €80,000. The rest of the gift will then be taxed at 30%. But this is just dangerous half-knowledge. There is still easily a thousand missing that would have to be paid monthly to Ratze

Then you would have €40,000 x 30% = €12,000. You first have to have that amount available in cash with this income, and there is always the risk that the tax office will even assess a higher value,

Even if one were to wait until October to make the purchase, and still had the money for the incidental acquisition costs and the gift tax, then for 2 older buildings, significant reserves for necessary repairs should also be planned.

Moreover, the right of residence does not expire when the two move into a nursing home. The limiting factor is death and not the move-out.