No need to engage your husband every time he calls, but your sons are still part of that family. My sons still visit their dad's' families, and keep in weekly contact via phone. The boys having Easter at your sil's doesn't have to mean that you are reconciling with your AH. Any way you could do your Easter thing and then let your sons have a visit with that side of the family for a couple of hours?
Lots of times in laws worry that they won't be able to see kids after a divorce, and it causes friction because they think the only way to fix things is to try and keep the two people together, or to help the other spouse fight for custody. Making sure the kids have contact with their dad's side of the family might alleviate some of those concerns and allow your in laws to stop seeing you and your hubby as a package deal.