This the price Sarah had to pay for helping to save her brother.
No, Sarah clearly says at the seance that
[spoiler]she's angry that Carolyn pretended to be her,[/spoiler]
so she's never going to come to them again. For once, the writers had a watertight excuse.
Sometime after I've seen all of Dark Shadows I'm going to try watching the 1795-related stuff in the order in which Barnabas experienced it - first 1795, then all the episodes leading up to the seance, then the episodes after Vicky gets back. But in the meantime, I think that immediately after the seance
[spoiler]a) everybody forgot about David's problems because Vicky was behaving like such a fruit loop that she made David look positively normal,
b) Barnabas was initially so distracted by the fear that Vicky knew too much that he focused on that to the exclusion of the lesser possibility that David knew too much and also to the exclusion of the exquisite pleasure of torturing Julia - and then he needed her help and the whole relationship changed - and
c) without Sarah to remind him that Barnabas was a threat and without Barnabas threatening him, David lost interest in the theory that Barnabas was somehow evil, and made it easier for people to forget that he (David) had been strange for a while.[/spoiler]
I must admit, however, that it was still a shock when
[spoiler]David ran into Barnabas's arms after Adam let him go.[/spoiler]