Seeking a small (I think it was green with ivy on the cover) children’s book about a little girl who is not allowed in her sister’s room without permission for fear she will make a mess. She gets in one day and does make a mess while playing, but then cleans it up. If I remember correctly her sister is angry when she realizes her little sister went in without permission, but then forgives her when she sees how tidy her room was left.
Could it possibly be Sarah’s Room by Maurice Sendak?