Manga No, You Na.

From Shinmakoku Scanlations :It was a life changing incident… Kinaho lived by herself until the day her boyfriend, Akito, suddenly brought home two young boys. The brothers were Akito’s relatives and had just lost their parents in a tragic accident. A very serious 2nd year middle schooler called Touma, his 5-year-old brother Haruhi, and Akito start their life together in Kinaho’s house. A series of confusing days gives way to the beginning of their new relationship—.