Translation:Are your parents also throwing a party?
kalas tends to be more food-centered and more likely to be for children, whereas fest tends to be more grownup and maybe more likely to involve dancing. Some combinations are used more often. födelsedagskalas 'birthday party' is more common than with -fest, kräftkalas 'crayfish party' is much more common than the version with -fest (although in this case, kräftskiva is the most established version). barnkalas 'children's party' is a set combination, and barnfest is rare. If I'd describe a 'samba party', I'd probably call it sambafest rather than sambakalas. But there's a lot of overlap.
"Are you parents going to throw a party too?" Seems to me Duo is being a bit picky here. And if I rephrased it as "Are your parents going to be throwing a party too?" would it make a difference? Don't get me wrong, I love the site and appreciate all the work you do, but at times Duo can be pretty annoying. As can we learners, no doubt about that. :)