I don't think your sis is a late developer... She sounds like
she's suffering from mild autism... She's not
MAD...
My step sister is autistic (really serious kind)... She can't talk
clearly, and she's like SUPER hyper-active and doctor had
instructed us to reduce as much sugar from her diet as we can
(that's why no more sweets and chocolates) cos sugar will make her
even more hyper active.
She cannot take simple instructions properly, half the time I
really wonder if she really register what we say, but when you
least expects it she does the correct thing with good effort.
One more thing... She's really really good with the computer...
Anything computer related, she's a whizz. Though I have not tried
if she could break passwords... But trust me... She's very
intelligent...
Read up on this web, maybe it can help you...
http://www.autism.org/contents.html
No matter what, I still love my step sister, cos it is not anyone's
fault that she's born that way... She's a loving person by nature,
just that genetics played a joke on her. There's no cure, so the
best way is to understand what she's suffering from, and making it
easier for her and the family to live with her condition, and
hopefully provide a nurturing environment for her to grow
up.