I think you've answered your own question here. You state that you don't really like to be around your mom that much, and that your mom and sister are closer. Also that your mom has made an effort to be around more since you've complained.
I think your mom is doing her best to give you the time and attention you and kids need, but also your space. Its easier to hang around people you have things in common with and get along with. If you want more time with her, then seek to reconcile your relationship so you really enjoy her company and then I think things will even out a lot!
Here's my experience. My mom and I are very close, but my older sister and mom have had a strained relationship. (possibly because she's more melancholy and has my dad's temperament and I have more of my mom's temperament). However over a year ago my sister was diagnosed with an aggressive ovarian cancer and underwent 3 major surgeries and a round of intensive chemo treatments. I went down to stay with her at my mom's and help out during the surgeries and the first 2 of 6 chemo treatments. I honestly thought it wouldn't work out for me to leave and have my sister and my mom getting on each other's nerves. Especially at such a critical time. However they worked through things and now are MUCH closer. Not like my mom and I, but my sister now enjoys going to visit them and talking on the phone, and asking my mom to come help her plant things in her yard etc... My sister is cancer free now and back to her normal life, but the relationship is now stronger with my mom, even though she isn't as needy.
I don't mean to point all the blame at you, I'm sure your mom has some work to do to make things right with you, but make a commitment to work things out with her and have a strong relationship!
Best wishes!