Who should I go with?

My mom wanted me to go to the stores with her to go shopping for gifts for my brothers tomorrow. Then my sister in law asked what I was doing tomorrow and if I need to wash. She let's me wash there at her house and we chill and talk. I told my mom what time she'll be going she said around 10 and my sister in law around 11. Then when I told her about maybe going to my sister in laws I kinda shut her down nicely because she has to get my lil brother from school around 3:30. My mom doesn't have a car so she won't make it back in time to get my lil brother and my sister in law can't take her since my niece gets out at 3:45 and it's a distance. I really need to wash but then I feel so bad that my mom and I haven't really spent time together. We are low on money so it helps alot my sister in law let's me wash at her place. I would go Wednesday with my sil but she's going to visit her mom and my mom works tomorrow. I feel bad I turned my mom down but then again the washing can help. Who should I honestly go with in your opinion?