Yeah that's awful that she made you feel bad about it!
If you can't avoid bringing Heidi, just make sure she doesn't touch your cousin. Chicken pox can be spread through the air (because the virus is also in your respiratory system) through coughing , sneezing and stuff so defintely don't let her get anywhere near anyone with chicken pox until she is vaccinated.
Shingles is the same virus, but it's only present in the nerve and in the little vesicles (blisters that are present in the rash). So it's not contagious unless you touch the rash, and you'd have to be pretty unlucky to get it.
So like I said - if you want to bring Heidi, I think it would be fine, just as long as she doesn't touch your cousin then I think there would be no problem. Otherwise, enjoy the time alone