Need Help Messenger chat

hey guys…need a small help from u guys..i guess most of u must have come across this situation in life.

i like a girl. we dont call each other much. but we chat a lot. we use facebook messenger. but the problem is his sister keeps on hacking her account n reading all messages..her sister is in intelligence..its like very easy for her to hack things…can anyone advice me any messenger where we can chat and there is no trace once the message is read and it self deleted..i cant call much due to certain limitations..so need a chating software which is highly secured …please advice.