Me [28F] with my sister[32F] asks me to be Maid of Honor. Do not have a good relationship. What to say in speech?

Hi-

I'll try to make this short. My sister has always been a difficult person to get along with. I'm the younger sister and she has always been really rude to me, always picks on me and judges me and overall I just feel uncomfortable to be around her. I really hate saying this but I don't like being around her and try to be around her as little as possible.

She got engaged and asked me to her maid of honor (basically because she had no one else to ask). The way she even asked me, was weird and didn't even seem like she wanted to.. she mentioned at dinner in front of my mother (why is this is importnat will be explained later) and I felt like I had to say yes.

There are no members of the bridal party, just me and the best man. One example of her inconsideration is when we went shopping for our dresses I was studying for a big exam and my sister gave a big huss and fit because I didn't pick out my dress that she wanted yet altho the wedding was months and months away. I told her that I would go but I was under a time crunch because I was studying for an exam. When I took the trip home (2 buses and a train), she sprung it on me that we would be looking for both my and my mom's dress. I had no ride to get home until they were done. I mentioned that I wish she had told me we were looking for both dresses and she flippped out and said that I ruined her day.

What I'm trying to say is that my sister is a big pain in the butt and really difficult to get along with. I don't want to be her maid of honor but I feel like my mother would be very upset with me if I wasn't. I am fine with going through the motions but I am at a lost about what to do with the speech that I have to make. I'm really not sure what I should say because for as long as I can remember, my sister and I have not had a good relationship. I need to say something though. Do you have any tips on what I should say during the maid of honor speech?

tl;dr: I do not get along with my sister but I am her Maid of Honor in upcoming wedding. What to say in speech?