Konggumi

The male protagonist confesses his feelings to a woman he believes is his friend’s older sister. After falling for her, he discovers that she is actually his friend’s biological mother. Despite the complicated situation, he finds himself wanting to meet her again. Original Webtoon

I thought she was my friend's older sister, only to find out that she was actually his biological mother...? But by the time I found out about it, I had already fallen in love with her. "I know that she's my friend's mom, but I want to see her one more time."