Sometimes i ask myself if those mangas aren't written by teenagers who haven't fully yet grasped relationships because who the heck would see some 13 year old as a love rival ?
Really hope the relationship with Aramiel to stay platonic at best, getting really sick of those low key pedo stuff in manga.
A lot of tradition sets the minimum age of marriage for women as 12. Keep in mind that this is a culture w/o public schools, harsh living conditions, etc.. Low life expectancy (e.g. 25) would not be surprising. In which case, a thirteen year old girl with very pronounced wifey behavior would be a potential rival.
However, given that Grey doesn't even consider it, I would guess that marriage that young is possible, but not common. Fifteen or older though? Yeah she would be ready for marriage.
People should understand that culture vary (especially in the past). It's fine to criticize cultures (I sure do), but different standards of living (life of expectancy, poverty, need to work, etc.) need to be taken into account as well.
All that said... "Today's 'Daddy' as tomorrow's lover!" is loathsome.
But (based on various spoilers posted here), apparently it's not an issue here. Thankfully.