Financially helping MIL RANT

How common is it to help inlaws or a single MIL? I'm having a VERY hard time accepting that my husband doles out money to his able-bodied mother at least every other month, most recently, footing the bill for her to go on a Hawaiian vacation with her friends. We have a child on the way and my anxiety is at 100%. We don't help my side of the family, so it seems very one sided & im tired of her pulling the "single mom" card- she's 60 years old & is only responsible for HERSELF!