We all love our sisters and wish no harm upon them, yet jump into bed with a different girl every day. You wouldn’t do anything to hurt her, yet cheat on your wife/partner repeatedly.
You would protect her with all you have yet you hit your misses over and over… Your sister is the apple of your eye.
Mum always said the way you treat girls is the way guys will treat your sister. Respect them, treat them right and don’t let momentary satisfaction cost your sisters lifetime happiness.
Keep her close to your heart and let that guide your judgment with the ladies.
Also How your sister sees you treat your partner will be the way she thinks she deserves to be treated.