ok here's the thing - i don't think wendy is into piss or would let stan piss in her/on her (MAYBE after many many pleas). she does indulge in his kink though, making him piss himself or going on a walk with him when they're pet playing. but she gets really shy when he asks her to piss on him!! it took her a while to even be able to release herself in such a situation
i think there are kinks they have that the other is not into (for wendy it's incest and for stan it's piss) but they're happy to see the other turned on so they go along with it