Is it permissible for a mother to give her son and daughter a larger share of her inheritance to help them marry, and would she be committing a sin by doing so?

Parents must be fair among their children in giving gifts, according to the hadith of Nu'man ibn Bashir: "So fear Allah, and be just among your children." There is no difference between the mother and the father in this regard, and it is forbidden for her to favor one over another unless everyone agrees. A distinction must be made between spending (nafaqah) and giving gifts (atiyyah). Spending is based on need; for example, if someone needs to get married, the father marries them off, and he is not obligated to give to those who are not getting married, because that falls under the category of giving gifts, not spending. Therefore, if you are about to get married and need money, your mother may give you what you need, and she is not obligated to give your sisters the same amount she gave you, nor does she need their permission. Your other sister should only be given to when she needs it for marriage.