I haven't tested this out but to make sure the dialogue only appear in this scenario, you'il have to edit the chapter script in Exalt, the script for Birthright chapter 15 has a similar condition (dialogue only if Corrin and Kaze have an A-rank support or above).Hey, I wanna add some dialogue between Kana and Possessed!Scarlet in Revelations Chapter 23 if Corrin S-supported her. I've got the dialogue and such all fixed up but I can't seem to figure out what to do to make it show up. I know it's done in Exalt and such but beyond that I'm bewildered - every time I try it I just get the generic line.
The function to get the support level is ev::UnitGetRelianceLevel($0, $1) where $0 and $1 are the units you want.
It seems to associate ranks with numbers (0 for no support, 1 for C rank, 2 for B rank, 3 for A rank, it can go above 3 so 4 might be S rank but there's also A+ so you should test it).
Again, this is just what I've understood from Birthright 15's script and haven't tested yet so it might not work.