It's not always about suing right away. The lawyer advises on what is legally possible and how it will impact the situation.
Professional advice is certainly never a mistake, although I had recommended the obvious approach: briefly consulting the notary who drafted the agreement between the two, which is the subject here.
The notary knows exactly what he intended to prevent with his contract and what it means if one party does not adhere to it.
Consulting a lawyer would mean explaining everything from scratch; the notary already knows the situation and comments on his own draft, possibly even by phone as a free, factual piece of information.
Otherwise, involving a lawyer always means a hard break in communication between the parties and an escalation of the situation. After that, there will be no more family talks, and the conversations between children and parents will probably noticeably change, because there is always the risk of inadvertently revealing the legal assessment/position of one side.
As already said, we all know almost nothing about the real details; even understandably only knows the part she is supposed to know, and only one-sidedly.
I find it hard to accept that one party spent the money and did this and that—that's too simple for me, even though as an affected person you might feel that way at the moment.
Moreover, it is the mother of my children, and despite understandable anger over possibly stupid behavior, she is still the mother I am dealing with legally. Since the children are already older, it would be a great idea to "distribute" this property appropriately to the children; a notary also has suitable ideas for that. The advantage would be that a delinquent payer would be doing something to their children, which probably wouldn't happen; so this would absolutely be the direction I would think in and which can be a largely amicable solution.
This would also further ease the apparently still existing "relationship" between the parents, and the adult children could take responsibility for it rather than just acting as toothless tiger mediators.
What I cannot understand is the fact that the father does not pay child support for the adult children directly to the children themselves, but to his ex-wife, the mother (and even deducts something for the property, which is legally not permitted).
I have seen this several times now and always wonder why the support isn't paid DIRECTLY to the children, who are already mature enough to resolve the parents' problems.
That is exactly what I would have changed immediately with the 18th birthday, as appropriate. The support belongs to the children and must never be a bargaining chip for the parents! Why do adult children have to collect the money owed to them from the mother and thus live in such an unnecessary, uncomfortable dependence on the parents?
I really feel sorry for you , because you are somewhere in between and want to help but simply can't. I can therefore only advise you to keep as much distance from this topic as possible, since understandably the in-laws' views are also biased!
You should better completely exclude these issues from conversations with them.