To narrow it: (human) actors, not people appearing as themselves (eg, documentary); and I'm looking for the longest list of siblings from one family, not groups of different siblings.
The Wayans family is usually a good source for this sort of thing. I'm Gonna Git You Sucka (1988) stars no less than 6 siblings: Keenen Ivory Wayans, Damon Wayans, Kim Wayans, Nadia Wayans, Marlon Wayans and Shawn Wayans.
[And as a corollary, Dance Flick (2009) must be holding some sort of record too. I've counted at least 11 different members of the Wayans family in the cast and crew.]
The film '101 Dalmations' featured no less than 200 dogs from around a dozen trainers. Several of the trainers provided dogs from litters of up to 13 dogs, all of whom would have been siblings (e.g. brothers and sisters).
It may interest you to know that they needed this number of dogs because no single dog could be used for more than 2-3 weeks because they grew too fast and stopped looking cute and that even those from the same litter required the services of a dedicated makeup artist who was on hand to make the dogs all look alike!
You may also want to consider "Phase IV". The film dealt extensively with a colony of thousands of ants, all of whom would have been sisters.
The 1980 western The Long Riders stars four sets of actor brothers as the real-life sets of brothers:
- The Keaches: Jesse James (James Keach) and Frank James (Stacy Keach)
- The Carradines: Cole Younger (David Carradine), Jim Younger (Keith Carradine) and Bob Younger (Robert Carradine)
- The Quaids: Ed Miller (Dennis Quaid) and Clell Miller (Randy Quaid)
- The Guests: Charley Ford (Christopher Guest) and Robert Ford (Nicholas Guest)
It also features an uncredited appearance by Ever Carradine, daughter of Robert Carradine and niece to David and Keith Carradine.
Additionally James Keach's son, Kalen Keach, is cast as Jesse James's son Jesse E. James.