I remember another manga I read with a similar plot, don't know if it was hentai or not, GF left MC for brother and then came back to MC because of issues, then he has to deal with her being abandoned by brother that, again after some chs, gets into picture to mess around, kinda shitty NTR feel overall, it was ages ago an some unconnected flashbacks are coming to mind all together now.
I love how the situation gets tense as if she should mean anything to him other than his brother's GF. . .