I reserve the term "trendy" for names without history / meaning; they pop up out of nowhwere and are therefore likely to leave quickly.
Traditional names that are popular are not "trendy" for me.
So
Alix and
Tristan sound perfectly lovely to me. They really do well together, as a sibling set.
Tristan brings out an almost chivalric quality in
Alix. I keep thinking of lances with sharp tips when I see
Alix next to
Tristan. Fun.