Snapping!

My 6 year old JRt, Polly, has allways been a mardy little madam, but recentlky since she hasnt really been worked, she has gone worse. NOt with me, she has never snapped at me, but it is all the other pets, and everyone else, my mother, sisters and their chiuldren, the list is endless, she will just snap as soon as they come near me, she even will charge them when they come in the house.
Now i know it's not as she is old, as she is in the prime of her life, but she hasnt been speyed (she is booked in for a few weeks BTW), she was allways to nervous, the vets didnt want to chance sending her into shock, but know the risks of not speying her are worst. Anyway, she has allways thought i was her baby, and wont have it any other way. I mean we have allways had other dogs with her, when she first came we had one of our staffies, the at the time 10 year old Mutley, and Polly was never that bad, with her or the old cats, and she was never that snappy. But when Mutley and the old cats died, she sort of found her feet, and by god do all the younger pets that we now have pay for that.
So any tips on what to do to stop her snapping, and being aggresive?