Unless your sister and her kids are living with your mom -- surely there are times when sister is not at mom's?
That's when you go. Period. I would not have my kids around her. If your mom is guilting you with stuff like, "But the cousins should all play together etc," well, that's hogwash. Just smile and tell your mom that the kids' schedules for school/activities/whatever have changed.
Don't expect your mom to do anything like ask sister and her kids over at another time different from your usual time with mom. Mom is clearly not going to do that and might even try a passive-aggressive move like inviting you over and then inviting sis and kids, and letting the fur fly between you and your sister.
I would be concerned, if I were you, about your mom and whether your sister is establishing some control over her. It sounds as if your mom may be grieving this death in the family, and sister has stepped back into mom's life at a time of need, so mom feels grateful and is falling on sister's shoulder, so to speak. Your sister may be manipulating your mom. You don't give enough background here for me to know for certain but....is it possible that sister is ingratiating herself and her kids with your mom, at a time when she knows your mom is vulnerable and craving reconciliation? Could your sister be doing this for ulterior motives of some sort -- getting money from mom? Revenge on you? ("I have mom now and you don't!") It's something to consider. If you think that's what's happening, you may need to enlist the help of other siblings or other relatives whom your mom trusts, to sound out what sister may be trying to get your mom to do for her. If you ask, your mom is going to brush it off as you being suspicious and jealous of your sister.
Meanwhile, no, don't let your kids go over when she and/or her kids are there.