"I believe the dj is implying that Fushiguro has feelings for his stepsister, and he feels guilty about carrying those around while being with Itadori. He doesn't want to show his dark sides, he only wants to show him his best sides and protect him. What Itadori saw wasn't a memory but Megumi's subconscious mind, and Sukuna was trying to make Itadori feel as if he was a replacement.
In reality, this are just Megumi's unresolved feelings. That's why Satoru told him that he might hurt Yuuji, because he was just giving his good sides and not being honest. I believe he loves both his stepsister and Yuuji, and I don't think that he is thinking of her while having sex or being in this relationship, but more like... he didn't get the chance to say how he felt before she went into a coma, all that is left for him are the feelings of wanting to monopolize and be with the only person who care for him, childlike emotions turned into lustful desire, I think.
But well, that's just my take on it. Feel free to make your own guess. Also, I think that at the end, Itadori realised that he can't force Megumi to come clean with feelings like these so easily, because he knows it's not going to have a desire outcome, since all he does is to give and give, therefore, he is going to have to make him receive his love back."
Jujutsu Kaisen DJ - Wrap Around