I have a new kitten and my "old" kitties hate her, what do i do to get them to get along??

i just got a 5wk old kitten and my 6 mth old absolutly hates her, to the point that she wont come downstairs if she sees the new kitty, i love them both and i want them to get along,, also, the "old" kitty wont even use the box, i dont want her peeing all over my house, im at my whits end, please help me