Do i tell my sister she betrayed and hurt me?

ok, i've never done this before but i need to act and i'm losing my mind trying to figure out how to go about this.

to make a very long story short, i was a foster parent to my niece and nephews who were sexually abused by their previous guardian. i have two children of my own. about a month after i took these kids in, i found out they did to my kids what was done to them. i found another family for them. i couldnt keep them after this. i have not spoken with them or any of my family involved. these kids denied everything and the family they live with now doesnt believe they did what my children said they did. to say that i was hurt by this reaction and what was done to my kids is the understatement of the century.
well, my problem now is that my sister, who is my best friend, became friends with my niece on facebook. she knows what my family has been through and i feel extremely betrayed by this. I feel that her accepting her friendship is saying that what happened is ok and that she just chose that side of the family over my kids.
she does not know how i feel about this and i think i have to confront her. i love her very much and dont want to hurt her. i dont know how to bring it up or what to say. it is a very sensitive subject for us.

i hope this story is clear, i am very upset and in a hurry. any advice would be appreciated. thanks.