I have an autistic sister - this is a developmental disorder that impacts her social interaction and communication.
Well is there a medical cure for autism?
No; there is therapy to help mange some of the behaviours but no cure.
So how is the therapy done? 
The therapy is used to help autistic people learn to correct their behaviours - like looking people in the eye when they speak, or teaching them epathy skills.
Empathy skills ? How is this approach applied?More so ,what's the origin of autism itself,is it hereditary or genetic?
Autism isn't hereditary, risk factors are things like drug and alcohol use in pregnancy, but there are environmental causes too. 
Yea the environmental causes,how does it translate in autistic behavioural patterns?
The environmental causes are very controversial; there are suggestions that the MMR vaccine caused a risk of developing autism.
MMR full meaning please?