Inheritance: seeking answers from professional lawyers

The agreement signed by the mother on behalf of the child should eventually be deemed invalid. If it is signed when the child is underage, the child's mother, as the guardian, gives up the inheritance right on behalf of the ward, which harms the interests of the ward. At the same time, the parties to the inheritance distribution agreement are related, and they must know the fact that the agreement harms the interests of minors, which is obviously malicious. It is an invalid civil act of malicious collusion to harm the interests of a third party as stipulated in the Contract Law and the General Principles of the Civil Law.

If the child is signed after adulthood, without the authorization of the child, the mother has no right to act as an agent, and the child does not ratify the agreement, which is invalid for the child. If grandpa didn't make a will before his death, he should distribute it according to legal inheritance, and his children have the right of subrogation to grandpa's property. As a widowed daughter-in-law, the child's mother can't see from your description that she has undertaken the "main" obligation to support her father-in-law, and should have no inheritance right to grandpa's estate.

As for litigation, fill in a complaint to the court and take other heirs as defendants. If other heirs claim inheritance based on this agreement, they will overturn the above agreement on the grounds that it is invalid, and claim redistribution according to the legal inheritance share.

In addition, it seems to involve the inheritance of the father's property. For that property, the possible inheritance relationship is very complicated. Without children, the death order of grandparents and fathers can't be analyzed.