Is there anything due from a woman if she swore not to attend her husband's sisters' weddings nor give them anything, but then went to them and greeted them before the 'zaffa' (wedding procession) and gave them a sum of money without attending the wedding in the hall?

If you swore not to give them anything, and then you gave them a sum of money, then an expiation for breaking an oath is incumbent upon you. This expiation is feeding ten poor people, or clothing them, or freeing a slave. If you cannot find any of these, then fasting for three days is required, and it is more prudent for them to be consecutive, based on the verse: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended of oaths. So its expiation is the feeding of ten needy people from the average of that which you feed your [own] families or clothing them or the freeing of a slave. But whoever cannot find [or afford it] - then a fast of three days [is required]. That is the expiation for your oaths when you have sworn. And guard your oaths."