Merrily, We Tumble Along

Meet Koshiro and Minato! When their parents remarry, the energetic third grader and easygoing high schooler suddenly find themselves sharing a home as brother and sister. There's just one little hiccup: neither of them is quite what the other imagined! Between their very different personalities and the inevitable growing pains of becoming a family, life under the same roof is sure to be anything but predictable.

(Source: J-Novel Club)