small new building + expansion of existing house - assessment

What can I say? Normally, not really. But one annoying neighbor is enough. I know, that doesn't help you

that is checked by the chimney sweep when he comes to measure.

Did I understand correctly that you are buying your sister's 12.5% share and that you are being gifted the mother's 75% share? Before you are 100% registered in the land register, the financing will not work, and then it depends on exactly what kind of right of residence is recorded. My suggestion is to divide the property and for you to build a second independent house and finance it independently. The mother is likely to live for a few more years and can pay out your 12.5% share, and with the property you have equity.

And in any case have the property divided. With all access ways etc.
Nobody knows what the relationship will look like in X years.