Is it permissible for my husband to borrow a large sum of money to bring a housemaid for his financially well-off parents, while he is under financial strain and I am in need of money to prepare for the needs of my coming child and my own personal necessities?

The obligatory maintenance (nafaqa) from a husband to his wife is what suffices her in terms of food, housing, and clothing, according to the circumstances of both spouses. He is not obligated to give her extra spending money beyond that. The expenses for preparing for the child after its birth are incumbent upon the husband, as are the expenses for prenatal care. If the husband is providing for his wife's maintenance in a befitting manner, then there is no blame upon him for spending on serving his mother. A righteous wife should help her husband in honoring his parents and maintaining ties of kinship. The husband, in turn, should combine this with being good to his wife.