It's kinda vague, i only know the plot of first half of the manga, so there's a couple, the girl invited the guy to come to her house, and the guy meet the girl's big sister, the sister approved their relationship
The next day the guy come to visit the girl's house but only to find the sister, the sister let the guy come in, and then the guy see sister's phone ringing and see that's from his girlfriend, so he got the call just to find his girlfriend seems like to fuck some other guy on the other line, the girlfriend thinking the one who get the phone is her sister, and it seems the big sister knows that her sibling fuck with another guy, so the guy just silently shock on the phone, the sister whom see it surprised too, and kinda blame herself, so the sister let the guy to fuck her

That's what i knows, i forgot how it end, and i kinda vague that this manga is one shot or story in tankoubon

Thanks for the help, and sorry if my english kinda bad, not my native language

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I'm already find it
It's manga by Momi, you can search it in hentai.cafe, it's tankoubon