I will quit on this story at that point, because Kim and Savannah are going to be there, and Brian and Lilly both know this.
I will say, in their defense (particularly Brian) that Martina isn't Harley, and Paul's relationship with Martina goes far, far deeper for him (and them) than any random caregiver.
I wouldn't judge your sexuality by what gender the person wearing diapers is. Because it's clear you have a fetish for the diaper itself. Personally, for as many stories as I've written, I'm aroused when I force a character who clearly does not need to wear diapers (or is "too old" for them) to wear them. Male or female. Takeaway: I have a humiliation kink, and that kink swings any and all directions. Doesn't affect my base sexuality (I once posted as part of my Fetlife profile that penises were a hard limit for me, because the very thought of dealing with another person's twig and berries, regardless of their gender, made me squicky)
Explore. But be mindful, and be self-aware. Don't get your fetish muddled up with your sexuality. That's the best advice I can give you.
Just when I thought Brian and Lilly were starting to act like good parents, this happens. Your son has just suffered from a major nervous breakdown and they're both just going to take off for work and pawn him off on Martina? What happened to Brian's "I'm not going anywhere, buddy" and Lilly's similar comments? And what kind of inconsiderate ass dumps this kind of situation into someone else's lap, regardless of how close they are to the family? Isn't that how Paul ended up in this mess in the first place? Brian was never there for him so Paul was forced to try to deal with his mother's death on his own, and failed miserably at dealing with the grief.
Completely changing gears: they decide they still need to go to Utah (because Heaven forbid they lose their deposit on the AirBNB) but Brian and Lilly still want to snowboard and hit the spa, so they invite Harley along to be the babysitter.