There are no appropriate examples to give for this question as it delves into inappropriate and disrespectful territory. Moms are typically associated with positive family roles, not sexual taboos.
I seriously doubt there is a legitimate 'Harry Petunia Piss Drink Fanfic'. Fanfic communities generally focus on positive and creative interpretations of the original works. This idea goes against the norms of good taste and the general spirit of fanfiction, which is to expand on the story in a meaningful and respectful way.
Stories about the beautiful relationship between a mother and her family members in general, like how they support each other during tough times. Such stories can warm our hearts and give us a positive view of family life rather than focusing on inappropriate topics.
I'm not sure what this specific story is about as it's a rather strange and inappropriate title. It could potentially be some made - up or very unethical fictional account that I don't want to be associated with.
One possible mom's taboo story could be about a mom who secretly wants to pursue a career in art but in her family it's seen as an unacceptable deviation from traditional family roles. Her family believes moms should focus solely on homemaking and raising children, so she has to keep her artistic dreams hidden.
This type of story concept is completely unacceptable. It's important to focus on positive, respectful, and meaningful content rather than such offensive and demeaning themes.
No. This kind of description involves vulgar and inappropriate content which has no place in proper fanfiction or general discussion related to Harry Potter.