Is spending on family members obligatory when they are in need, or not? And how can abstaining from that be justified, especially with regard to one's mother? And is it possible to demand marriage expenses, similar to one's siblings?

The maintenance of wealthy children for their poor parents is obligatory by consensus. However, if the parents are wealthy, nothing is obligatory for their child except by way of recommendation and preference. The question implies that the parents are not in need, but you should gently apologize to them, explaining that you need the money, and it is permissible to use tawriyah (double entendre). It is not obligatory for a father to marry off his wealthy son; rather, it is obligatory only if the son is poor and unable to afford it. If your brothers were poor at the time of their marriage, then it was obligatory for the father to marry them off, and you do not have the right to demand marriage money from him. The forbidden favoritism is pure favoritism without a reason. However, if your brothers were wealthy at the time of their marriage, then you have the right to demand an equivalent amount to what they were given, out of fairness. If they do not respond, then entrust your affair to Allah and do not abandon their dutifulness.