Move her hand in such way that she lowers the strings from her shoulders and then the nightie should drop down. It is easier to move down the string from her left shoulder, then you should ty to move down her second string, which is a bit more tricky and then she must take her right arm in the middle so that the nighty can slide down.
Hi, John Rambo, I can see no problems to continue the game, when Becky is totally naked. This happens two times: first, when she has taken off her nightie - as far as I know, she continues by herself in dressing with her mother's square-dance dress. The second time she is naked at the hey-loft, after her pussy has been licked. Then she will get fucked. This happens also nearly automatically.