Spanish uses ser (es) and estar (está) differently:
- ser (es) is used for inherent characteristics, things that are seen as part of someone’s nature or identity.
- estar (está) is used more for temporary states or conditions.
Being realistic and sincere are seen as character traits, not temporary moods. So we use ser:
- Mi psicóloga es realista y sincera.
(My psychologist is realistic and sincere.)
If you used estar, it would sound like you’re talking about a temporary way she is being right now, which is odd here:
- Mi psicóloga está sincera hoy. – sounds like Today she’s being sincere (implying maybe she usually isn’t), and even that sounds unnatural. In general, sincero/a goes with ser.