What to do when your ex hurts your kids feelings?

Ok,I have two boys from my first marriage and a daughter with my husband now.My husband and I always try to treat all of our kids equally-when we do things,buy things etc.My ex gets the boys (15 and 13)on the weekends.Sometimes he makes excuses and does not get them.I can deal with that-I try not to say anything derogatory about him in front of the boys,even when I know their dad is lying about why he cannot take them.This past Sunday my boys came home and my oldest was mad.He told me he had found out from his stepbrother(exs new wife