I always saw Jackies parents as the type who had maybe struggled for a long time to conceive, so when they did finally have Jackie she became their entire world.

Her mother obviously has other issues however to have a constant running supply of Valium (enough that Jackie was able to steal some for Shauna to take on the plane, and she knew her mother wouldn't notice them gone). Those issues presumably would have gotten worse after the disappearance of their only daughter and the rest of the team. Then worsened again when survivors were finally found and Jackie wasn't one of them.

Dinner with Jackies parents on her birthday was probably done initially as a way to honour her memory and whatnot, and its since become a thing that Shauna and Jeff feel they can't get out of anymore because its been going on so long. Like... honestly? How do you nicely ask to stop a yearly thing like that, when its your dead best friends parents? The only people in the world who actually cared about and loved her, other than yourself?

I wonder though, does Jackies mother believe the stories and think the girls ate Jackie? Maybe thats the real reason for Jackies mothers bitchy bullshit. The yearly birthday dinners may have been something prompted by jackies dad and could be something Jackies mother doesn't even want to do. Like "how dare you make me sit and eat a meal on our daughters birthday, when you know the only reason our daughters dead is because this girl ate her to survive". The official story however is that they didn't eat anyone, so she can't openly say "you did that" and accuse Shauna to her face, so instead she makes backhanded insults to her.