I completely agree with all the posters on here (you need to set bigger and better boundaries, dont go, etc.) but I personally would take it 1 step further on 2 counts.
1. you dont need to protect just your boys, you need to protect YOU and your husband too - the idea of the police thinking you are complicit in harboring a fugitive is NOT absurd. Please dont take this to mean that you shouldnt protect your boys first, my only thought is that, if you justify your refusal to go to your mother's house because of the boys, if for some reason, they cant be used as the reason any more (they are away one christmas/they have grown up, etc) your mom sounds like the type of person to use that against you and make you feel guilty for not coming yourself. And since you already have boundary/respect issues with her, I would hate to see you think that you should acquiese to her just because they are not around. Thus I say, this needs to be a FAMILY decision.
2. For your own safety and peace of mind, I would suggest cutting off contact with this woman/family. I do not mean to be presumptious about this, but it sounds like you are coming to that conclusion yourself, and I think it might be the best for you and your family (based on what you have posted here. Each post shows more and more callousness in regards to her decisions and how they affect you). Again, I dont want to sound presumptuous, and I typically am the first one to give people 2nd and 3rd and Nth chances, but this situation is truly bad. It is NOT rude behavior toxic, it is not make-you-feel-bad/uncomfortable-everytime-you-see-them toxic (as most of the posts on here are). This woman has put you and your family in danger by harboring a fugitive, and is doing it again. This is a pattern of behavior supported by a strong belief system that she is showing and WILL NOT CHANGE. I wouldnt expect her to take you and your family into consideration into her decisions, as she clearly has no concern for your safety.
Please, please, please stay safe.