For people who have little interest in the twins, don't worry. You will
Don't worry too much about the panther or Ms marvel. They'll come soon enough. Marvel doesn't have to adapt all their characters straight away after all!
As for the origins, some people have talked about terigenesis (how the Inhumans got their powers) or that maybe the twins are descended from two superheroes from WW2.
Getting back to my first point though, trust in joss. The twins haven't had much of a role in comics in the last 10 years which is why I think some people have the 'don't care' attitude but they really are great characters and great Avengers. If you read some of the stories when they were Avengers you will realise why they are definitely Avengers material not X-men material