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Illusion or not, Jean's feelings about it were her own. Mastermind conjured up an illusion that tried to convince Jean that she was happy WITH the kids and Scott. Jean struggled against the illusion of a happy family because of her REAL feelings. Mastermind's illusions brought out the truth–that she felt trapped by future kids she had nothing to do with. That scene was her fighting back, to get out of the illusion. And I'll give you that this was before the trip to the future. BUT, when they got to the future, she and Scott both acknowledged their feelings for Rachel as their daughter too - multiple times, and specifically during the rescue scene where the narration describes Jean's maternal feelings towards her), despite not raising her. And remember, for the whole time they were in the future raising Cable, they were ALSO taking care of Rachel's comatose body for 8 YEARS. (Issue 3 of that mini shows Scott and Jean had her on life support the whole time, and tended to her regularly, until she died.) So all those years of taking care of her and Nathan both, albeit differently because she was old and he was young, have led to lots of evolving feelings for Jean and who she considers family, I'll bet. You said earlier that Jean got to bond with Nathan as a baby and doesn't owe Rachel anything? Well, if it wasn't for Rachel bringing Scott and Jean to the future in the first place, she would never have gotten the chance or the time to bond with baby Nate. So yeah, Jean and Scott both owe Rachel big time, for getting 8 years of mommy bliss family time with Nate. If someone gave you that gift, wouldn't you love them more/feel closer/feel a sense of obligation to them more too?
There are also other stories, like the X-Men Forever mini (the one written by Fabian Nieza and drawn by Kevin Maguire, not the horrible Claremont series), where Jean was sent to the future and found Rachel there. She also expressed her maternal feelings for Rachel in that issue. And then there was recently when she came back in X-Men Red's annual, and who was the first kid she felt she needed to see and protect? Rachel. Not Cable. Why? Because Rachel is her and Scott's daughter whereas Cable is Madelyn and Scott's daughter. Not that she doesn't love Cable–because like you said, she spent some time raising him – but Rachel is a biological connection. Telepathically, they've psionically bonded their minds and the mother-daughter bond between the two seems to be intimate and intuitive (And this was all AFTER she went to the future to raise Cable as her child. Nice try yourself.)
And to your earlier point in a previous post where you state:
Pushing a child out of your vagina doesnt make you a mother and its quite dismissive and insulting to adoptive/step parents who raise and love a child. - I kind of agree, if there were no nuances to be considered. To that end, just because a woman didn't raise her biological kid for their entire life, it doesn't mean she's not still that kid's biological mother. I have a biological dad I didn't meet until I was 14 yrs old and we've spent my adult years getting to know each other. My step dad raised me from birth, and my biological did not. But they're both still my father. (and to your other point, no - it does not feel more like an uncle to nephew scenario - it still just feels like a dad to son situation)