I can't exactly remember what the relation of the little boy and the high school guy was (I think he was in high school), but I think the gist of the story was that the aunt was too busy with work to take care of the child and so began the story of the high school buy babysitting the kid occasionally for his aunt.
The only thing I seem to remember fairly well is that one day the little kid wanted hotcakes and the cousin made him the ones that looked like it was from the box. (Like this https://media1.popsugar-assets.com/files/thumbor/SgZ6LcZW9pSxoWQUwlqcyiYBIa0/fit-in/2048xorig/filters:format_auto-!!-:strip_icc-!!-/2016/05/04/137/n/1922398/f189276af72fa3ac_d7b4e923_edit_img_pinterest_post_image_file_41165299_1462230703_Japanese_Pancakes_PINTEREST.jpg) And the mom tried reallly reallly hard to replicate it (she half thought in her head that the high school cousin se the bar too high since the hot cakes looked exactly like the boxed image) and she couldn't do it in the end but the little kid was like,"It's ok, it doesn't look the same but the hot cakes are still yummy." or something along those lines. I also think that the title of the manga was fairly short.

Not sure about this, but is it Flat (AOGIRI Natsu)?

Yes that is the manga! Thank you so much! 😀