When an Ashkenazi Jew prays with Sephardic Jews, should he sit during the repetition of the shmone-esre and during kaddish?
Question
Can an Ashkenazi praying with Sephardim sit during the repetition of the shaliach tzibbur or kaddish?
Answer
Dear …!
Those who normally stand during the repetition of the shmone-esre and during kaddish should do so even if they pray in a place where this is not an accepted custom. They don’t have the problem of going against the local custom, because the Sephardim also stand sometimes during shmone-esre and kaddish. That said, you should try not to be too conspicuous as you stand contrary to what most other congregants do.
Source
Sitting or standing during shomne esre and kaddish isn’t so much a matter of custom. Rather, it’s a personal stringency. Therefore, everyone should continue doing whatever his ancestors used to do before him. See the Shulchan Oruch with the Remo commentary (section Orach Chaim, ch. 56, §1), Mishnah Berurah §§7-8