What next?help !!!!!!!!!!?

my brother in law sort of touched my ,well he did and livin in the same house with them feels like hell.his wife my own sister told me not to tell a soul.am in fear that one day he will want to have with me.i badly want to tell mum but am failing to do so,i made a promise to my sister that i wouldn't. please help with advice.what should i do next?