Considering I usually always S-ranked Lissa, I can't believe I've never seen this before. The resemblance I mean.

Peri is royalty? I thought she was just a noblewoman, which is anywhere from just below to way below being anything considered royal.

Just a reminder that royalty =/= nobility. Nobility is the mediaeval equivalent of middle class, and if her family has enough influence or titles, a politician. Maribelle was very high up for nobility as a Duke's daughter, and probably had some distantly royal blood. Chrom's most socially likely marriage option really. The distinction still applies though.

For another example, in A Song of Ice and Fire, Ned Stark is not royalty either, though his nobility is a factor in being his country's functional equivalent of a prime minister.