Unexpected deaths are the worst, really. Not to say that deaths you can see coming are completely harmless, but it's difficult to know how to react. I don't think there's a right or a wrong way to react, it's just how you react... nobody can judge you for it
When I found out that my dad had died overnight after being admitted into hospital for some, as far as I knew, chest infection or whatever, it completely shook me and I still haven't processed it to this day. Even all of your relationship issues (if there are any) just don't really matter, it's just a suckerpunch really.
I hope you & your family get through this together and are OK, just make sure you can keep in touch and offer each other support is all I can advise.