This achievement is 10G and the achievement description states that you must "defend a womans honor".

You must wait until sequence 2 to begin fight events.

The first available Beat Up event in Florence is called A Woman Scorned. It is available after you complete the first DNA Sequence. When the next sequence starts, you will automatically be speaking with Annetta, your familys live in servant. After that conversation, turn left and you will see a woman glowing white. Speak with her to find out that her husband is being unfaithful to her and wants you to beat him up. Accept the mission and when you have control again, follow the yellow crosshairs until it disappears and becomes a green circle on your mini-map. Activate your Eagle Vision to locate the husband who will be glowing yellow. Simply beat him up as you did your sisters boyfriend and the achievement will unlock