eh~...maybe bcoz they are brother and sister? i know what you are feeling,I want them together too but just think. It is not just about the society but when siblings have kids the most of the times the kid have some kind of a disorder and the mother knows that and doesn't want her kids to live this kind of misery. You can get bass the people badmouthing you but to see your own child sick is something unbearable.
Spoiler...
But they are not related by blood. The mother knows that and still being such an asshole knowing they both love each other and are now grieving the death of their father. The least she could do is tell them they are not blood-relatives and accept that they are in love... but then again the manga would probably end right there and there's still like 10 more chapters left. *shrugs*
i srsly spent half an hour thinking about that and i can't find something that can justify the mother. Except for the want the society will say there isn't anything else. you can say that she wanted them to be just siblings and her plan was ruined, i don't know. A person's soul is an abyss. But i believe that that was only something the mangaka did to bring tension to the story.
Why can't the mother let them be lovey-dovey together? Isn't she play to much comedy? She doing this for convince the "couple" to not be together... I feel so sad for them. Want to see more, and to know WHY the mother don't want them together...