Family feud between my sister and i please help me?

Ok so my 16 year old nephew was staying with my family for a few weeks when my sister was on vaction my wife caught my daughter kissing my nephew in her room now my sister is blaming my daughter for manipulating my son into . They came to me and told me they have feelings for each other my sister called my daughter a slut and a to her face i was so applaud at my sister for acting like this so i told her i never want to see her again. My daughter is upstairs in her room crying about the whole thing she is only 15!!!!!!!!!!!! My wife slapped her and kicked her out of the house. My sister leaving threatening messsages if my daughter comes near her son again she would have me arressted for harrasement!!!!!! WTF I am just as upset with the whole thing as she is my sister is angry with me for letting this happened and being to calmed about it. I'm a very low tempered man so its very hard to make me angry but if you ult my family then we have a problem. My nephew and daughter realy want to see each other i told my daughter that she can't see him because of the whole souation with my sister. I am the youngest out of six children and my sister is the second oldest but we were always close we never fought like this even though i was the baby of the family. We also have a family gathering coming up this weekend and she is going to be there.