can people w/ peanut allaergies eat Pistachio Nuts?

Just try with few pistachios once.

Despite the name peanuts aren't real nuts, however they have a similar structure and so you can still be allergic to nuts if you have a peanut allergy. So it's very possible you could be allergic to them.

I'd discuss this with an allergist (a doctor that specializes in allergies)

Peanuts and tree-nuts are very different foods. While it's not uncommon for someone to be allergic to both, it's also common for person to be able to eat one but not the other. If you aren't allergic to other tree-nuts (like walnuts or almonds) you could probably eat pistachios as well. But no real way to know.

Maybe. But not worth any sort of risk taking.

I know people who are allergic to all nuts including peanuts. I know at least one person who can eat peanuts and pistachios but cannot eat walnuts and pecans.

I make pesto every summer in large quantity, it is one of my summer go to dishes for potlucks and parties. I usually use pignola (pine) nuts but depending on if I know someone has allergies I can also make it with pistachios or walnuts or even with no nuts at all depending on the allergy status of whoever is going to be eating.

Probably, unless you are also allergic to pistachios. (Peanuts and Pistachios are very different foods.)