There is no official canon relationship in FE: Awakening. ChromxSumia is teased quite frequently, yes, but so is ChromxF!Robin throughout the story. In fact, Sumia more or less becomes an extra after chapter 13, while Robin still remains important (and continues to remain very close to Chrom).
You choose whichever spouse you want in Awakening. It's only logical that Lucina would hesitate to kill her own husband/mother; if she remained gung-ho about killing someone who's related to her by blood or by marriage, how would you feel about her after that?
Personally, I don't like M!RobinxLucina, because to me, it has a lot of elements of Wife Husbandry going on. My preferred pairings for M!Robin are Cordelia, Olivia, and Emmeryn, but that's just me.