With or without a building savings contract (interesting model)

Wow, you guys have gathered quite a decent amount in the family. How do you handle the repayment? Monthly as it fits at the moment? We are also considering accepting support but would prefer not to "pull someone into it" from the family. But within the family, the interest rate is of course the best, 0% :-D

So we pay 1.5% interest on 100k to my mother-in-law. Which is okay, this way we had a nice sum to show the bank and we don’t feel bad about it. Since my wife has three other siblings, it was fair and no one can complain.
My parents gave 20k at 0% interest with no conditions. (Eventually to be repaid sometime, I also have three siblings)
We repay the 100k to my mother-in-law however we want. So far, in one year, 20k repayment, since we had one year without repayments at the bank. We want to have this amount fully repaid in 9 years to then save for 5 years before the 180k need to be refinanced. Which will hopefully work, as my wife will work again for one or two years before child 3 and 4 arrive. Then she will stay home again for 2-3 years and then go back to work until retirement at 75 :D

I’m still relatively new to the topic. How do I calculate how the monthly payment changes with the remaining debt if the interest suddenly goes up to 6%?

Take a repayment calculator, calculate what remains as the outstanding debt after your 10, 15, 20... years and then simply take 6% of that amount (amount times 0.06). Of course, this is then only the interest and no repayment yet.

In any case, it depends on the personal situation. Everyone has to know for themselves how they can/should proceed in order to look calmly into the future.