Hi there. Sounds like you're having a rough time. As a custodial parent and a child of divorce I just wanted to give you my thoughts.
It is very normal for children entering their teens and throughout their teens to resent you as a 'step' parent. Remember that their world has been turned upside down, children of this age often still dream of their parents reuniting. Their 'CP' may not necessarily be to blame for their feelings (trust me, my stepdad and I had daily arguments for at least 3 years after he and my mom married. Sidenote - now I love him to death!). All you can do is continue to try and hope for the best. Do not push them away because ultimately this will cause problems between you and Dad.
Also remember that as teenagers their personal lives will require much, much more of their time. They want to be free to spend time with their friends when they're not in school. Tell your husband to try and work with them. I know that alot of times I had plans with friends on Friday nights, so my dad would come and pick me up around 10 on Saturday morning and we would spend Sat and Sun together. As they get older, schedules will get more hectic, so try to implement a plan of working with the kids to schedule visitation now and hopefully that will encourage them to ask for altered visitation time instead of just deciding not to come at all. Also, try to involve yourself in their lives. Your family should try to attend events that they are involved in, such as ball games, school events ie 'PTO', school concerts, scounting events...they may act like they don't care if you come, but in the end it will show them that you care and that you are committed to being a part of their lives.
Also, at some point during each visit have Dad set aside time to do something just with them. It could be as simple as going to McDonald's for lunch or playing basketball in the driveway. Anything to show them that although he is remarried and has a new family, they are still very important to him. This should also give you and your son a break.
Tell Dad to call them often, send them cards, letters, anything to show that he's thinking of them. When they don't visit, call and let them know that you all miss them and you love them. Children thrive on love and attention and if you continually show them that, they will eventually respond.
Once again, these are just my thoughts, but I hope they help. I wish you all the best in the years to come!