Lottie I am very sorry for the loss of your sweet Budgie! I would be very upset at your sisters friend also and upset for the loss itself!
I don't see why you should not get another Budgie, it seems you love your Budgies very much.
Maybe next time you can put your Budgie and his cage in your bedroom and lock the door while friends are over so no one can take your Budgie out without your permission.
It was not your fault though, you did the right thing telling people to leave your Budgie in its cage, sometimes people are thoughtless as this person who let your Budgie out was.
Again I am sorry for your loss! :-)