Firstly, you may want to get a waterproof mattress protector. They are reasonably priced so at least that solves one of your problem. Secondly, if I were you, I will request her to help and change the bedsheet and that includes washing the stain pajamas. Let her get into the process of cleaning so that she learn and undestand the need.
I will not try diaper on her because that will encourage her to continue wetting. Is she well toilet train during the daytime? Please reduce amount of water few hours before bedtime.
Not sure if you want to bring her to see a doctor or would it help. I came across a site which helps you potty train. View Start Potty Training.
There's another mummy in this forum who has similar problem as you. You might want to share experience with her. Take care...