Keeper of Night, So you're asking "If Chrom has a C support with Sumia, but Sumia has an A support with Frederick, will Chrom marry Sumia?"
And the answer is Yes. The game only checks to see Chrom's highest support. He marries his highest leveled support after Chapter 11. However, if his highest level support has an S rank with someone else, Chrom cannot marry that women.
Ex.
Chrom C support with Sumia. Frederick A support with Sumia = Chrom marries Sumia.
Chrom C support with Sumia. Frederick S support with Sumia = Chrom cannot marry Sumia.
In Ch.11, Chrom's last potential wife becomes playable. So don't fret if Sumia, Maribelle and Sully are all married.