Well, have you tried discussing it with your step-kids? You might find they do want to see you and they might set your worries at ease.
You could write them a card, saying what they mean to you and that you want to stay in touch.
Or talk to XW about scheduling holidays a few times a year. I know you've got your sons, but can you not have all 4 kids sometimes, or take your sons on one holiday and your step-kids on another?
Sure, you wouldn't see them all the time, but I'm sure you still could a few times a year.
