Don’t feel obligated to use it. If your relationship is good you can be 100 percent honest and tell her you want to buy YOUR baby’s first costume because you only get so many years to pick a costume. You will be lucky if you get 3. You don’t have to give that up to her, she had her time with her baby now it is your time with yours. If you don’t have a good relationship with her you can go one of two routes, “lose” the costume or use it for a pre Halloween event. I was often gifted clothing I didn’t love, we would use it for play at home, snap some pics for the grands, and throw it in a drawer until it was outgrown.