Well if nothing else, the title isn't misleading. Sondra Stillman and Kari Foxx do indeed play sisters, and it is indeed sinful that they and the rest of the cast are not better served in this tiresome incestravaganza.
The titular siblings, seen at home relaxing on a waterbed, fantasize the sexual encounters they've seen between various family members, and dream up some new ones while they further amuse themselves with each other. That's the only link for the subsequent couplings, all of which take place either on the waterbed or on the two other sets.
Not that the scenes in and of themselves are bad. Nina Hartley and Marc Wallace are delightfully lascivious as the family's amorous aunt and uncle, and Stillman and Foxx do well in their scenes together (including a memorable "love beads" sequence— and with brother Billy Dee. The majority of the video, however, is so negligible that one has trouble remembering the sequence of events. Surely there must be enough spare plots floating around the industry that one could have been inserted into place. Compared to a true "incest epic," like Hannah Does Her Sisters or Taboo American Style, Sinful Sisters comes off as only a weak sister.