My sister had unsafe sex with her ex husband on the 1st of july after that she had her period 10 days later on the 11th of july.she then had sex with her partner after her periods (11 days later) on the 22nd of july.who might be the father?

It had to be with the partner in the latter encounter.

Please remember sometimes "sexual histories" aren't always 100 percent reliable. (Mistakes can be made, either consciously or unconsciously). However, if she feels certain she had her "normal" period after her first sexual encounter and she has fairly regular periods. Then it would be most likely she became pregnant with the next encounter (11 days after period), a prime time for ovulation.