Is it permissible to build an apartment for one's father with the available money, knowing that the son's work is unstable, or to build a part of it, taking into consideration the noble verse: "You and your wealth belong to your father"?

May you be rewarded for your solicitude for your parents. The greatest act of filial piety towards them is to strive for reconciliation between them.

If that is not possible and your father is poor, then it is obligatory for you and your brother to spend on him and provide him with housing. You are not required to buy him an apartment as you did for your mother, but if you can do so without harm, then do it.

As for care and service, it must be provided to him if he needs it, even by hiring a servant, or by marrying him off if he needs a wife. Marrying him off may be obligatory for the children in this situation.