She is manipulating you as she is jealous of your younger child. How disabled is the younger child?
I agree with 3rd, make sure to have a special time on a regular basis just with your older child.
Stay calm, make sure you tell her you love her, praise her for good behaviour and be consistent about punishing the bad behaviour.
I have a 6yr old special needs child. So I understand they are a handful.
Can you spend some alone time with your 5yr old. Take her out...just mommy and her? Once in awhile or once a week so she knows she's special.?