Yeah, assessing one's own family members would be a pretty clear conflict of interest and would certainly be unethical
In this case, as with doctors treating family, it's probably not a good idea. Maintaining one's objectivity when dealing with family is practically impossible.
I should imagine that while forensic psychologists would take a close interest in a family member's case they would not be actively involved. I think this is more for films where the protagonist gets involved and exonerates the accused.