Behavior Question

[ashley, whistle, nikki]

I need a little help with a problem I've been having recently. We just found a new kitten, and she has adjusted well to our household, but our other cat, Whistle (3 years old) does not like her at all. Whistle has become skittish with everyone, will not come into the house except to eat really quickly, and she hisses at the slightest disturbance. It is unfair to Whistle to disrupt her 3 peaceful years with us, but my parents said that we will have to give Nikki away, to a good home of course, but no one in the family wants to part with her. Is there any way that I can bring the two together? Help!
Ashley