I am widowed and would like to know how I should notify my deceased spouse's parents of my upcoming wedding. Is it necessary?
If you and your former spouse had children together, then it's definitely necessary to let their grandparents know you're remarrying. But even if you didn't have kids, it's a thoughtful gesture to let your in-laws know about your new marriage, since their child was such an important part of your life. If you're comfortable doing so, call them with the news. If not, writing a letter is fine too. There's no "requirement" to do this, but it's really the right thing to do, especially since there doesn't seem to be a reason not to tell them. If you're nervous about their reaction, remember that being honest is your best bet. Then concentrate on your new life and happiness with your fiance, and allow your former in-laws their feelings, whatever they may be.