Hajimete no Hito wa, Kareshi no Otouto.

Tsugumi is a girl who went through a tough breakup with the first boyfriend she ever had as a student. Several years later she finds herself being loved unconditionally by her senior coworker Shinobu. Day in and day out Shinobu fawns over her and drives her mad with his incessant loving. She feels at times like her body can take no more and is happy in her relationship with Shinobu until the day they decide to go back and visit his family home. What a shock when the person to come out and greet them is someone she never thought she'd ever see again, Rio. He was her very first boyfriend and the one she gave her innocence up to and is apparently... Shinobu's little brother!? Rio, the one who thanked her for letting him take her virginity, appears in her dreams again and again. Although they broke up long ago, he continues to play around with her and make her body ache for him.

(Source: Manga Reborn)