I love this, just like other said: A lovely cheat-isekai feel without all the fighting. It interchangably brought both tears and laughter on my face. This would be high on my suggest to read list.
PS: Love the little niece, she reminds me of Naru from Barakamon. Looks the same and is just as...
If the mom has the bloodtype AO, the O is silent, and the father has BO, again the O is silent, then using Mendelian genetics there is a 0.25 chance that the child has bloodtype OO.