'My Sister Always Fights With Her Boyfriend'

Please pardon my English. Is one years now that my sister introduced a boy to me as her boyfriend, since then I have gone to their place several times to separate fight and it seems it is getting worst but they don't want to end the relationship.

I warned my sister never to near the boy. To my surprise I will see the two of them again and my sister will say everything is now ok.

Please what legal action can I take to separate the two of them?

source http://www.nairaland.com/6845910/sister-always-fights-boyfriend