Is the word sincero the same in Brazilian and European Portuguese?

Yes

The meaning, spelling, and grammatical function of the word "sincero" are identical in both Brazilian and Continental Portuguese. The only difference lies in pronunciation. In Brazilian Portuguese, the vowels are typically more "open" and clearly articulated. In Continental Portuguese, unstressed vowels tend to be more "closed" or even reduced, which can make the word sound slightly shorter or more muffled to a Brazilian ear.

Brazilian Portuguese Examples

1. Eu sempre tento ser sincero com as pessoas. (I always try to be sincere with people.)
2. Ele me deu um conselho muito sincero. (He gave me a very sincere piece of advice.)
3. Eu preciso ser sincero com você sobre isso. (I need to be sincere with you about this.)
4. Ela é uma pessoa muito sincera e direta. (She is a very sincere and direct person.)
5. Para ser sincero, eu não gostei do filme. (To be honest, I didn't like the movie.)

Continental Portuguese Examples

1. Eu tento sempre ser sincero com as pessoas. (I always try to be sincere with people.)
2. Ele deu-me um conselho muito sincero. (He gave me a very sincere piece of advice.)
3. Preciso ser sincero contigo sobre isto. (I need to be sincere with you about this.)
4. Ela é uma pessoa bastante sincera e direta. (She is a quite sincere and direct person.)
5. Para ser sincero, não gostei do filme. (To be honest, I didn't like the movie.)