Well, it's clearly inappropriate. In our society, there are certain boundaries and values. Family relationships are sacred, and suggesting something like this goes against all that is considered decent and proper in human relationships.
In this type of fanfic, the Infinity Stones could have a very different effect in the Harry Potter universe. For example, the Time Stone could be used to reverse dark spells or resurrect fallen wizards. However, the Stones' power might also be limited by the magic rules of the wizarding world. The fanfic could delve into how the characters from both universes adapt to the presence of the Stones and how they use or misuse their power.
Definitely. The Infinity Gauntlet comics came out earlier and laid the foundation for the concepts and characters seen in Infinity War. There are some differences, but the comics serve as a precursor.
Avengers Infinity War is largely based on the Infinity Gauntlet comic. Many key elements and plot points are drawn from it, but there are also some creative adaptations and additions to make it work for the cinematic universe.