Resident female cat does not accept new kitten Help!!!!!

My wife and I have an almost 5 year old neutered male cat and a just over 3
year old spayed female cat. We just got a female kitten who isn't spayed as
she is not yet old enough. The male was a bit wary at first but has
accepted her with great lengths - watching her, giving her baths, letting
her play with him, etc. He was the same way with his older sister (he gave
her baths and she actually suckled on his nipples). The problem is with the
older female. At first she couldn't be in the same room and if the kitten
got anywhere near her she hissed and growled - she never tried to physically
harm her but rather backs off and runs and hides in her favorite hiding spot
if the kitten gets too close. Also the male has become very protective of
the kitten and intervenes. The older female has pretty well always been
wary since we have got her - she generally hides if anyone else comes over
and even with us isn't very affectionate (she will lick my face if I'm
holding her and has developed a love for a back scratcher being rubbed
against her back and belly) - however, she is very attached to her big
brother and has always followed him around. While we are out of the house
we leave the kitten in a spare bedroom with the door closed with some food,
water, and one of our litter boxes so she doesn't get into too much trouble
and too allow the older female a break to wonder around the house. When we
are home we also take the kitten into the computer room in the basement for
a bit so the older cat can walk around the upstairs and main floor and eat
water. The older female otherwise has been hiding out under the bed in the
other spare bedroom (her favorite hiding spot). At night the kitten sleeps
in our room. We have now had the kitten for 1 night and 3 full days and
things do not seem to be getting any better for the older female. How long
will it take for her to warm up to the new kitten (if at all). I feel
really bad for the older female because this seems to very stressful for her
especially with her older brother protecting the kitten from her. Is there
anyway we can speed up the process. My wife gave the kitten a bath to see
if getting the smell of her birth mother off of her would help but it
doesn't seem to. Any replies would be appreciated

Thanks,

Dan