In general terms from my experience I'd say it's never good to assume anything that hasn't been established on screen in some way - in fact it can often be dangerous.
As far as Elizabeth and Roger being brother and sister goes, as Midnite pointed out, one specific reference so far to that was cut from the NBC airing (and thus the DVD), though it was restored on the MPI VHS. (I'm not even sure it's been established that Elizabeth is David's aunt.
) But as to what personal info we may learn down the line about any of the characters or the family, it's been so long since I've watched the '91 series that I don't honestly remember...
i've never seen 'buffy' or 'angel' or anything.but isn't it typical that an air of mystery surrounds the goings on for awhile before the presence of vampires is revealed towards the climax?
Certainly not on Buffy or Angel or even any other vampire themed series in recent memory. The existence and involvement of real vampires is established from pretty much the get go.