Buying a two-family house, feasible?

Be sure to go to the bank. The €30000 debt is not exactly what makes you a good borrower. Possibly refinancing. And settle the house matter with your father. Whoever owns property has all the rights and obligations attached to it.

over 24 months

24 or 12? Parental allowance is not the same as parental leave. Be careful!

Or are you planning the Elterngeld plus? Then your wife would have to have earned well beforehand to reach the 900.

24 or 12? Parental allowance is not the same as parental leave. Be careful!

Parental leave 24 months, parental allowance as well
It's called parental allowance plus
The parental allowance is simply divided over 24 months instead of 12 months
Example
Family A receives 12 months of parental allowance
€1600 per month
Family B receives 24 months of parental allowance
€800 per month
In total the same but distributed differently.

The apartment building is managed by us children, as it was transferred to us a few years ago
Keyword inheritance tax etc.
Father continues to take care of the house as long as he can.
My brother and I are listed in the land register as owners, each with 50%.

Elterngeld plus is, simply put, about one third of your wife's net income before birth. How do you get to 900 euros? I just want to save you an unpleasant surprise, presumably you have already done your research properly or are using a provision that I do not know.

My brother and I are listed as owners in the land register, each with 50%.

Where do the rents from 5 apartments go? Why is there income? Your brother is supposed to be liable with the house for your debts? That’s where sibling love would end for me. Questions upon questions...