That's like one of the worst kept secrets in video games, though.
Hell, they almost flat out say that Lucina is Chrom's daughter in the dang reveal trailer.

If Robin doesn't marry Chrom, then her support with Lucina would basically be Lucina accusing Robin of trying to seduce Chrom which prompts Robin to say that she never found him attractive. Lucina becomes furious and demands that she fall for Chrom's charms.