What to do about sis?

My mom doesn't get to visit us often because she's older and doesn't care to fly alone. After a recent trip to visit her, my sister suggested bringing her home with us to visit. The main purpose was to see my niece in a play. She asked if I could help entertain her, and if my mom could stay at my house if my sister's house was too warm. I told her that of course I would. She also mentioned taking her to a casino since my mom had never been.

Sis and mom arrived home a day after us (Sunday). I didn't hear anything from her until Monday. She called and said that she didn't want to monopolize mom's time, and that mom could stay with me Friday night (after the play) and then I could bring her back Saturday evening. She then told me that she and her best friend took mom to the casino on Sunday. I was a little bothered that I wasn't invited, but hey, it was her plan with mom. I invited sis, mom, and my four nieces and nephews for supper, which I cooked. My 18 year-old nephew (one of the youngest) was the only one to offer to help clean up after supper.

I didn't hear anything Tuesday, so I e-mailed my sis Wednesday morning and invited everyone for supper again. At 3:30, sis called and said that my timing was good; she had to work and I could pick mom up for supper and bring her home (sis's house) after.

Friday night we all went out for supper and then to the play. My sis told my mom to pack everything and take it with her to go to my house. After the play, mom asked sis when she'd be coming back, and sis didn't give her a clear answer. She just looked at me and jokingly said, "when you're sick of her, send her back.". Saturday, when driving mom back to sis's house, mom commented that she was worried that she wasn't expected back so soon. I told my mom what my sis had told me on Monday. I then told her that if she wanted to go back to my house, that was okay with me too. She went back to sis's house. I asked sis and mom if they wanted to come for supper one night this week.

Sis called this morning and asked if I wanted to do supper one night before mom leaves. She said that she had plans with mom for Wednesday, but Thursday would be fine. Then she said that I could do supper tonight if I'd rather. I told her that I'd have to do some shopping (I'm cooking for 9 after all) and that Thursday would be better. I work, and wouldn't have a lot of time to shop and then cook for everyone.

So, am I wrong for being a little peeved at sis? If mom were having fun, I might feel better, but my mom aid that when she's with my sis, my sis is tired after work and they just sit and watch movies. I know money is tight, but there's so much here to do that's free that mom has never done.