my cat is aggressive toward her sister and her new litter

Question
hi, i have two female cats, myrtle and shiva, sisters who are almost a year old. shiva was in heat a few months back and sneaked out one day (we had been keeping her indoors because we didn't want her to have kittens), so she had her kittens three days ago and they're all doing realy well.
however, myrtle isn't impressed with these new arrivals, she hisses and growls at shiva and the kittens(its not constant, she stands over them for a bit evry time she's in the room, but it looks like it could get worse) am guessing its a fear/territory thing or maybe feels threatened by all the attention shiva is giving to these 'imposters'. myrtle was always the more affectionate of the two and would always look out for shiva and let her have her way...she still has loads of cuddles for us when she's not around shiva and the kittens. but now just the last three days she hates her sister, and am also worried she might kill the kittens if she gets a chance.
i've tried speaking softly to her to calm her down but it doesn't seem to work. maybe it will in time? shiva isn't being aggressive in response at all. if anything she seems a little upset and confused, and wary for the safety of her kittens.
unfortunately keeping them separated seems like it might cause more problems, shiva decided to give birth on the couch beside us so i moved her box up onto the couch as soon as she was done, now i'm nervous of disrupting/moving them to a different room in case she rejects her kittens.
should i banish myrtle from the sitting room/kitchen for the next couple of months?- she eats there and has to go through there to use her litter box- would this make her feel more left out?. or should i see how it goes if i move shiva and her kittens up to the bedroom?(which is really myrtles territory!)
i've been googling lots on cat behaviour but cant find anything for this particular situation. its such a nuisance, and really sad to see this change in behavior, i'm geting them spayed asap to avoid this happening again. the kittens are all going to good homes as soon as they're ready, i just hope the two cats can get along again as we've had a great year with them up until now, i dont want to have to give one away.
i would be so happy to hear any tips or advice you could share on this! thanks, amy :)))

Answer
For the safety of the kittens (as well as Myrtle's sanity), I recommend that you move the kittens to a more private area, or at least somewhere where you or somebody else can keep a close eye on them. That way, Myrtle can still have her area as well.
She feels they are intruding on her "turf", which is most likely why she is acting aggressive.
Cats are very territorial creatures, but it will most likely die down eventually. She may not adore the kittens as they grow into cats, but will most likely tolerate them.