Sorry to hear you are in a tough spot. I think all of Tony's suggestions are great.....particularly getting your nieces to engage with you....maybe play a card game or board game, if you have one, with them. There are online quizzes that kids/teenagers seen to prefer e.g. on pop music, film etc.
If they cannot be parted from their phones, maybe do something for yourself. Listen to a meditation CD, have a hot bath, give yourself a manicure, do some drawing or colouring. I find mindfulness very difficult but find breathing using imagery easier. An example is using slow breathing while picturing yourself in a safe place in your mind.
Finally keep posting through it here. This will pass.
Support to you.
DS