I don't necessarily have anything for pre-loss anxiety, but I did suddenly lose my dad. As in talked to him the day before and he was feeling a little sick and the next day I learned that he had died alone in his home. Without going into some of the sadder details, I can tell you what helped the most.

I felt complete in our relationship in the moment. I still have times when I wish I could share something with him. But, I felt like I had really vocalized the love I had for him. I love you doesn't really count in my mind, but being specific about how they make you feel loved and how you love them. And secondly, I had accepted/forgiven him for anything that wasn't in that love category.

That feeling of being complete was really nice to have after his sudden passing. It didn't take away the sadness and missing him, but I didn't feel the regret that I've heard others do.

So, my suggestion would be to express the specific love you feel for your parents and in laws to them and forgive or accept them in the other things. I hope this helps somehow or at least makes some nice moments in your family.