All perfect praise be to Allah the Lord of The Worlds. May His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
In terms of spending on relatives, the Sharia maxim states that one who has sufficient provisions and maintenance isn`t entitled to be spent on because he/she has enough. It is stated in [Moghni Al-Mohtaaj, vol.5/pp.186].: "The principle by which this issue is measured is considering the age of that relative, his needs and wants. This is in addition to paying the wages of a servant if he needed one, proper clothes and dwelling, doctor fees and price of medications."
However, if that relative wants to buy items of luxury, then one isn`t obligated to pay for that.
If your (Asker) father in law provides your mother in law with sufficient provisions and maintenance, then her children aren`t required to pay her any amounts. But, if her husband doesn`t provide what is enough for her, then her children are obliged to make up for that shortage, but they don`t have to pay for her trip to Aqaba or for Umrah year after year.
You (Asker) have to keep in mind that your husband must please his mother, but at the same time he has to give you your due rights. And Allah The Almighty Knows Best.