This. The death of a child is something that you don't really get over. The folks projecting in the post are really quick to judge this behavior but it's certainly anxiety regarding losing a child.

Idk what happened to your gf's sister but in my experience, when your adult child doesn't show up to work and her coworkers/friends call you to make sure she's okay, only to find out she's dead - that stays with a person.

That said, dealing with the grief is the only thing that's going to deal with this. You have to accept that you have no control over what happened nor will you have control over what will happen.

You need to approach this from a place from understanding the behavior and helping your gf help her parents deal with the root of the issue.

Coming into a family who has dealt with this loss is a huge undertaking. You will be the voice of reason but your gf and her parents may see you as uncaring. If you steer the ship gently into the realm of healthy behavior all will work out in the end, trust me.

Be patient, you are walking into a huge situation and judging by your OP, I'm not sure if you understand that. 3 years isn't much time to get over a loss of this magnitude. It does get better. At 7 years now my family has their moments but things are much much better.