Who is the man's Sister?

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."

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If Amanda is telling the truth, then Melinda is lying.
If Melinda is lying, then Vanessa is his sister.
If Vanessa is his sister, then she's lying about Eva lying.
If Eva isn't lying, then Amanda is lying,
Which would contradict the fact that Amanda's telling the truth.
That means Amanda is lying and could be his girlfriend or sister.
Eva must be telling the truth since Amanda has to be lying.
So Vanessa must also be lying, and Melinda is telling the truth.
Since Melinda's telling the truth, Vanessa isn't his sister and must be his girlfriend.
Amanda is the other liar, so she must be his sister.

Answer: Amanda