I'm sorry to hear your news. Maybe my experience can help.
My sister was married in 1999, and they took custody of his teenage niece, we'll call her J. She considers both of them her "parents". So they are gram/gramp to her kids now. My siblings and I consider her our niece even now, we are aunts/uncles.
They've since divorced and both remarried. A few weeks back J had a princess b-day party for her 2 girls. All sides of the family attended. Her biological mom and step dad. Her grams on her biological parents side of the family. My sister and her hubby, my other sister, my dad, my sisters x-hubby and wife and kids, his parents.
It was such a strange family get together and absolutely no tension. My sister and her ex each run their own businesses out of the same location and have for over a decade.
As with all relationships you get out what you put in. Although it is a painful time for you and the family. Keeping the lines of communication open is definitely worth the investment. IMHO.

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