Owain x Morgan, if you marry Emmeryn. Like, seriously, guys. Just because Emm has amnesia, it doesn't mean she's not related to Chrom and Lissa anymore. Do they just not care that first cousins have married each other? You won't let Lucina x Owain, don't let Owain x Morgan.
As a matter of fact, I think the same goes for the secondary children of Chrom's; they only really accounted for Lucina which is just pure laziness to me :/ Do better next time, Intelligent Systems. Do better.