Qitzur Shulhhan Arukh - Yalqut Yosef, Siman 56:11 writes (my translation):
מנהג האשכנזים כדעת הרמ''א, לעמוד בשעה שעונים קדיש וברכו, וספרדי שמתפלל עם אשכנזים, נכון שיעמוד גם הוא עמהם בעת אמירת קדיש וברכו, כדי שלא יהיה בכלל יושב בין העומדים.
The Ashkenazi custom, per the ReM"A, is to stand when responding to Qaddish and Barekhu. And, as for a Sepharadi praying with Ashkenazim, it is proper for him to also stand with them when responding to Qaddish and Barekhu, so as not to sit among those standing.
My $0.02: Perhaps Hakham 'Ovadiah is poseq this way since standing among those sitting or sitting amongst those standing could cause Mahhloqet. Again, that interpretation is mine and he does not write that explicitly. He simply writes "so as not to sit among those standing".
Rav Chaim (Howard) Jachter rules similarly (in "Sephardic and Ashkenazic Practice: Do You Sit or Stand for Kaddish?") for an Ashkenazi praying amidst Sepharadim (i.e. that one should follow Minhag HaMaqom):
In addition to conforming to Minhag HaMakom (which, in general, is the
proper practice – see Mishnah Pesahim 4:1) it seems disrespectful to Hashem if one
would exclude himself from a Kehillah which is standing in honor of Hashem.
Conversely, it seems appropriate for an Ashkenazi praying at a Sephardic synagogue to
follow the custom of the Kehillah to remain seated for Kaddish.