Gift / Semi-detached house / Land register

The property as a gift should of course remain mine alone, but the house will of course still stand on it. All the more important is that everything jointly financed is fairly divided in the end.

As far as theory goes. If you are attached to the property and do not want your sister to live next to your ex, you will have to pay him out. This is difficult for most people, so a sale is imminent.

That's true. But how else should it be arranged? He won't want to stay there just because of the proximity to family, and therefore it only makes sense that I stay there if I can somehow afford it.

As far as the theory goes.
If you are tied up in the property and your sister is not supposed to live next to your ex, you will have to pay him out. This is difficult for most people, so a sale is imminent.

If the properties are divided, then the sister is irrelevant in case of a separation. To put it extremely: if the ex then wants to live next to her, with her, or alone on the other side of the world, then that’s just how it is and no one else’s business. Trying to prevent something like that is childish. You can only regulate the financial ownership of the house, which does not at all mean he has to move out immediately. He can actually stay living in your house at first, if moving out would be "unreasonable." But that’s just a side note; these legal extreme cases are rare.

I think you will end up with a classic marriage contract that takes the €100,000 out of the accrued gains and regulates the retention or sale of the house. Beyond that, the community of accrued gains is supposed to remain? Will the man be involved in the financing and be entered in the land register?

If the properties are divided, then the sister is irrelevant in a separation. To put it extremely: if the ex then wants to live next to her, with her, or alone on the other side of the world, then that’s just how it is and nobody’s business. Trying to prevent something like that is childish.

Irrelevant in a legal sense, yes. Emotionally, not irrelevant at all. We are talking about the inherited property on the homestead of her family. Of course, in the case of a divorce, you don’t want the ex or some stranger living there. It would be equally tragic if the OP had to overstretch financially in the event of a divorce in order to not “harm” the rest of the family.

I think you will end up with a classic prenuptial agreement that excludes the €100,000 from the accrued gains and regulates the retention or sale of the house.

Inheritance does not flow into the accrued gains adjustment. The value of the property remains hers either way. However, ownership will change; he will usually be entered into the land register. In the prenuptial agreement, you can then set something like a right of first refusal for her and/or her family, should a separation occur.

Is this even possible in terms of financing if our partners basically share the costs as tenants?

From this, one can assume that only the sisters want to be listed in the financing and land register. The partners pay "only" rent.

Where is the advantage in the rental model supposed to be? The rent is paid with money that is subject to gains and moves from the left pocket to the right pocket, which is also subject to gains. The assets to be divided in a divorce do not change, with the disadvantage that the tax authorities sit in between and demand taxes on rental income.