Kitty doesn't take to strangers

Hi,
I have a cat named Shera, she is 1 going to be 2 yrs. old in April. I
got her from the shelter and the main issue she had was that she
didn't like to be around children. She was an abused kitty who got
her tail tugged on by some children before she was put into a foster
home. When my boyfriend and I adopted her she was 7 months old and we
were told of her past abuse and also told that she would not do well
around children and that it would take a while for her to warm up to
us. As soon as we got her home however, she immediately started
demanding to be pet and loved. She seemed to do so well with new
people, even jumping on their laps and letting them pet her. Then,
for whatever reason, a little bit past her first birthday she started
acting different when it came to strangers.
She usually hides under the bed and refuses to venture out of our
bedroom unless all the strangers are out of the house. Sometimes
she'll go on the bed but as soon as she hears a loud noise she'll go
right back under the bed again. If she happens to be out and about
and a stranger comes into the house she either runs upstairs as fast
as possible (that's where my bedroom is) or stands there and does a
long, aggravated meow and sometimes she'll scratch or growl. Not
saying that we have a lot of strangers coming in and out of the house
all the time but we do get the occasionally visitor.
My concern is that my boyfriend's sister is going to be moving in with
us and so is her three year old little boy. I'm worried to death that
Shera is going to wind up being traumatized by this whole ordeal.
They may be here for months and I don't want her to be afraid and hide
under the bed all the time. I'd like to give her the benefit of the
doubt and say she might take to them, but when they were here two
months ago for two weeks she spent all her time cowering in a corner
under the bed.
I have no idea what to do. I need suggestions and would appreciate
any advice on how to calm her down or at the very least keep her
mentally unscathed from these soon to be visitors.

Jenn