Main female is a student, neither loli nor milf. She has rather long than short black hair. She uses a cell phone (and old flip-flop model) to call her brother to come visit her at uni (or school? idk).
During the conversation she mentions another brother, it was like a manipulative strategy of hers.

After her brother (he is a little bit taller than she is, handsome) arrives they are all into love making indoors.

Next what i remember they are sitting on a bench outdoors and two friends (classmates) stand by to say hi. Wile sister introduces her brother to her friends her brother uses his opportunity to secretly finger his sister under that bench. That was a really hot scene. Btw artist's skills were nice, not only with drawing people but with backgrounds as well.

I read this manga before (long before) 2018 for sure and it still buzzes me that i can't remember the title. I beg your mercy with it.