Are pistachios likely to cause problem if allergic to peanuts?

Possibly. "peanuts are legumes, biologically unrelated to tree nuts. But there is high level of allergic cross-reactivity between peanuts & tree nuts, meaning that people with tree nut allergies are at increased risk of developing peanut allergies. If you're diagnosed with a tree nut allergy, your allergist will advise you whether to avoid peanuts as well." see comment for more info.