Is the money a mother gives to one of her children, which differs from what she gives to others, considered a religiously acceptable expense, given the differing circumstances and needs of the children?

Parents must be fair among their children in giving gifts. The details of this are that fairness in giving a gift means that all children are given, with the male receiving double the female's share. As for fairness in spending, it means giving each child what they need, and it is permissible to favor the needy over others. Therefore, what you took for marriage needs is permissible as long as you were in need of it and your mother was giving your siblings what they needed. However, giving you money to buy a house is not permissible unless all your male siblings are given the same amount and females half of it, because the need for housing can be met by renting. If your salary is not enough for rent, your mother can give you the rent or buy a house specifically for herself that you can live in for free. If all siblings, being mature, agree to the preference, then there is no harm, provided their consent is genuine.