Inroducing a new cat into home

I just took in another rescue. She's a exactly 1 year old, female Lynx point siamese who is preggers named Misty.

I have one other cat, a 1 year old female snowshoe siamese named Mittens. Mittens is very possesive and jealous and immediately hissed at Misty in the cat taxi.

We're currently keeping them separated, but I am wondering if this is necessary???? How can I make the transition easier for both and how can I get Mittens to accept another cat????

up date....Mittens is only acting more hostile....is there anything I can do to help the situation??

This link here helped me

http://www.thecatsite.com/forums/showthread.php?t=67321