Ya, the saying "wait", "stop", "no", that totally signifies she wasn't against it. Fear makes people shut down, and if I had a creepy fatass raping me, I would shut down too. The 'get it over with quick' thing is also pretty standard for rapes.
Moving in with your husbands parents after you have a kid in Nihon basically turns the wife into her in-laws servant. That said, if she had an itch she wanted to scratch, cheating with older brother neet is really a much safer option than neighbors or milkmen or etc.
I don't think this is rape so much as the 'my husband won't touch me after seeing me give birth' and becomming a workaholic, and I want to feel desirable again genre. It doesn't culturally translate that well... but I don't think it's uncommon here in the US as people seem to think. Also this is a NEET fap tag and there's dozens and dozens of versions of the same story...
...Well, I can understand it being tough to hold back, but he's kind of...well, not so much blackmailing, more so threatening her in some sense?
But for her to so willingly go along with it makes me wonder if she, uh, kind of wants it.