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Did anyone help you in making the two application that got refused? Did you provide evidence that mother collects your father's pension in Pakistan and she has a house in Pakistan to prove that she will go back after her visit?
The rules on visit visa have changed since 1994. If your mother and sister came to the UK what is the proof that they will return back home? They should consider swearing an oath through a lawyer/courts in Pakistan declaring that your mother and sister will return to Pakistan and that they have no intentions of remaining in the UK.
Your sister is dependent on your mother so both applying together