Who is the man's sister?

Question

A man escapes from jail with help from his girlfriend. Four girls are accused of being the man's girlfriend. His girlfriend is lying. Two girls are innocent and telling the truth. The other girl is the man's sister who is helping the girlfriend lie.
Who is the man's sister?

Amanda: "Melinda is his girlfriend."
Vanessa: "Eva is lying."
Eva: "Amanda is lying."
Melinda: "Vanessa is not his sister."

Answer 1

Ans:
Amanda is his sister.
Venessa is his girlfriend.
Eva and Melinda are truely speaking girls.

If Amanda is his sister, she had given false statement.
It means Melinda is not his girlfriend.
If Eva is truely speaking girl then Amanda is lying which we assumed earlier.
If Venessa is his girlfriend she had also given false statement.
Means Eva is not lying which is true in our case.
Finally Melinda said Venessa is not his sister which is true because we already assumed that Venessa is his girlfriend.

So in our assumption all consecutive conditions are satisfied...

This question can be solved by taking different cases but in all other cases all conditions are not satisfying.