Is this wrong please help

My older cousin is incarcerated and I have temporary custody of her youngest two who is 3 and now 1 and the youngest calls me mom and even starting to get separation anxiety and clings to me a lot and believes I'm her mom her mom is getting out soon what do I do is it wrong to let her call me mommy even though she has been doing it since 6mos even though I tell her I'm not her mom is that wrong I just want her to know I'm her cousin and not her mom but what's best for her I need help