T.K.
I think that this will certainly affect your son (and you!) for a long time to come. You can't control other people, but you guys at least should seek either professional or spiritual counseling to help you truly work through the issues that undoubtedly lie beneath the surface for both of you. Hopefully in time, his parents will come around.
Until then, I would write them a letter Start it with Dear Mom and Dad, then move onto why you are angry. Use those words "I am angry because I feel like you abandoned our family at a crucial time. I am angry that you blame me for having my husband's wishes, and not yours fulfilled."...etc. Then say why you are sad "I am sad that we have lost a much bigger part of our family than just my husband with his death. I am sad that directly or indirectly, you are keeping my step daughter from us. I am sad that you no longer seem to care about me as a family member. I am sad that my husbands death brought this on."...etc. Next: Fear. "I am afraid that my son is suffering. I am afraid that he feels abandoned. I am afraid that having to deal with this plus his dad, might make him have problems later in life. I am afraid I might not see ___(stepdaughter)___ again. I am afraid of losing her love over this conflict. Next: Apologize. Be sincere. Use the words "I'm sorry". Think of things you may have done that seemed hurtful to them, and how you may have contributed to the problem unconsciously. Finally. Tell them you still care for them as family, no matter what their feelings may be, and that nothing will change that. Tell them you know they have to grieve in their own way and in thier own time. Then tell them what you would like. "I would love to reconnect with you as a family" or whatever statement defines what you would like to see as an outcome. Send it. If you get a response, wonderful, if you don't, well, it may take them some time... Your open and honest communication about your feelings should help them identify with thier own feelings, and help them have compassion for you and for your "cause". And keep inviting them to birthday parties, etc. And don't take it personally if they can't attend. Explain to your son as well (if you feel that it's appropriate) that there is no way to hurry Grandma and Grandpa's grieving process, and that for them seeing you may just remind them of their loss at this time. Let him know that it is likely that they will come through it in their own good time, and let him know that he did NOTHING to cause thier withdrawl. Good luck!
May Love, Light, and Laughter be yours.
T.