how do i deal with nightmare?

Things or people during the day can cause tensions that cause nightmares. Eliminate the tensions and possibly the nightmares go as well. Simple meditation forms can help too. Let the child visualise another, positive ending to the bad dream or help visualise beautiful dreams of things they love. Just let them close their eyes and start a quiet conversation building those positive images they can take with them into dreamland. Let them direct the conversation with help of your questions as much as possible to give them the feeling of control. Good luck.

And wonderful you take such good care of your little sister when no one else does...

used to give my son an herbal supplement called "No More Monsters" Seemed to help after a couple of months.

Tell her a funny and nice tale before sleeping.