Yes it is. However it's not netorare if they are all in on it, it's not netorare if they all wanted it from the start. This is not netorare just lesbian wincest with a side of dick.
Wow. Hot...Netorare, huh? I dunno about that bc it kinda seemed that nobody stole somebody from another person. If Mami had a boyfriend, sure but I think it's inferred that she's single.
Certainly an interesting title. I will admit, seeing the NTR tag made me think of a scenario of little brother + older sister...After reading through it though, I'm still not entirely sure where the NTR comes in? Didn't feel very NTR-ish to me.
Still, First, he forces himself onto her while planting the idea for her to confess to her sister (Set-up for a 3P?), and when she's starting to doubt herself, he just kept pushing. Rather smart of him.
Heh, that ending though. So...It turns out that her sister was in on it, or rather, the one that actually asked him to do it, just so they could set up a lovely 3P relationship. How nice.