In a question and answer session of an interview someone asked Jullian Fellowes about a lady's maid for Edith. He replied that she did have one and gave her name - but explained that she wasn't seen on camera. There are, supposedly, many more servants besides the ones we see on camera and who are given lines. As JF said, if they had lines, "Then you have to pay them more!"
The single daughters did share a maid (Anna,) and Mary, as a married woman became entitled to her own. However, Edith as she is the only single daughter left, couldn't be expected to dress herself now could she?!
Just as one more note, the cast said in an interview that once Sybil (Jessica) tried to dress herself and came down with her dress on backwards!