My sister is a sl*t and a liar?

ok so me, my parents, and my sister moved to a country and my mom doesn't have a job yet and my dad is in the country where we came from and he's working there to support our family while my sister asks my mom for money and say that she needs it for something else like "my friends owe me money" when the truth is she's going to stranger's houses that she met online in this dating site called "questchat.com" and sleep with them and goes back home at 1 in the morning... she's 18 years old and doesn't have a job and isn't married yet... .... she always asks money and now we're having a big loss of money.... i want to tell my mom the truth but i'm afraid that my sister is going to kill her self or runaway and i don't want this to happen even though she's 18 years old (she already had runaway before and tried to kill her self; jumping off a building and such).... what can my mom do about it and how do i tell her? my sister has an anxiety/depression disease and if i tell my mom that she is being a sl*t, i don't know what my mom will do to her... at the same time, i want my sister to get caught and i don't wanna see her cry too....