Awful bridesmaid

So i asked my sister to be one of my bridesmaids. When i asked her i gave her a cute box with gifts in it and just said "i guess" to the question in it of if she wanted to be a bridesmaid. She has always been a person with kind of an attitude, but she has been nitpicking at everything. My wedding isnt until october 2020 but i am bridesmaid dress shopping early because one of my girls is in the Navy and has to leave and will be gone until the wedding. Anyway, i was aiming to shop this july and when i picked a date she complained that it was too early. I personally feel like it is never too early to buy the dresses, considering the shop im going to said they typically take 6 months to come in. She also complained because there were no dresses under $100 (which is an unrealistic budget). I made sure all my girls had jobs and had the funds to be bridesmaids. I just feel like she cant ever say yes to my suggestions without argument. I don't think I'm being unreasonable. I tried talking to my mom about this and her response was just "that's how she is". Which is basically telling me that i have to accept her crappy attitude because she is just like that. I know that she is just going to stress me out and make things difficult as we get closer to the wedding, but i don't want to kick her out of the wedding party. What should i do?