Appointment with the construction company, what do we need to ask?

I am not a tax advisor, but I think: if the land and house are considered as a unit, all relevant taxes apply to the entirety (?). If you are lucky, I am mistaken regarding VAT - but essentially the warning remains: what the tax office considers as a unit is not remedied by registering your fiancée in the commercial register of the property company and your brother-in-law in the construction company. If an identical "beneficial owner" stands behind it and has influence on the management and/or it can be derived from a number of similar cases that the split personality is actually a single one, then the tax trap will snap shut.

If you're lucky, I'm wrong about the VAT

You do. A land/house purchase (I include the general contractor contract and developer contract) is always a VAT disadvantage, meaning without VAT shown for the buyer.

You mean in the sense of "stupid for a deductible buyer, because it's included but can't be deducted"? - that would be irrelevant for the classic private builder.

I had looked at the imprint myself for both companies, and my tax advisor also reviewed the contracts.

I thought that in the worst case, real estate transfer tax applies to both but no VAT on the property?