Is it permissible for a father to donate 100,0 pounds to his two daughters who suffer from mental disabilities, to be divided equally between them, for their expenses in care homes or for whoever is entrusted with spending on them, while preserving the legitimate right of the wife, given the absence of any siblings to care for them?

You may donate all your money to your two daughters, or donate some of your money to your husband and donate the rest equally to your two daughters. You are not obligated to give anything to your siblings or other relatives. If you have children other than the two daughters, the general principle is to be just in giving to them, unless preferring the two daughters is due to their specific needs, in which case it is permissible. The greatest thing that safeguards children and secures their future is the fear of Allah and the righteousness of the parents.