Im sure youre parents love you and your children as much as the others. But for whatever reason, they choose to spend more time with the other family.
Unless theres no communication between the two of you, I suggest that you try talking to them. Its possible that they arent aware of how you and your children are feeling.
If your son really feels the need to express himself, he could write them a letter. This accomplishes two things. First of all, you could proofread it and make sure its good enough that youll still get your financial support and your parents wont be so insulted. Secondly, being that it will be in writing the anger wont be so apparent and theres no danger of him losing himself as he might if it would be a face to face confrontation.