If you were renting the house to a third party on the open market, let's say you would charge €2,000 a month.
So, if she stays in the house, you are "losing" €1,000 a month.
So she should pay you €1,000 a month rent.
You must do a written agreement.
I would suggest that she agrees in writing that the property will be sold after 10 years.
The alternative is that you could sell your half to her now for say €300k and she could repay this to you over the next 20 years. This could be difficult to enforce if you fell out.
Or you could give her a loan to buy your share of the house and you would secure the loan on the property just like any other mortgage. This would stop her selling the house without repaying you.
If you charge interest on the loan, it would be taxable in your hands. But the capital repayment would not be.
Brendan