Well, if she is "wiggling" it out, then you need to tell her she is doing better and give her encouragement. After a few days, then work on "let's get there sooner"...........getting her to do it on the potty is good.....even if it is late....you know the saying, better late than never.....in this case, not so much, but still it's a start.....
Start keeping a journal of when she goes potty......see if there is a time schedule.....like between 10-10:30, she goes poop........that way, you can make sure you are around her at that time and help her make her way to where she needs to be.
Also, let her pick out something small at the store she would like to have......tell her if she poops in the potty for 2 days, you will buy it for her, or for a day if it's cheap...........then increase the prize along with the time.......be sure to give her praise whenever she gives it a try.......talk to her about being a big girl like her sister..........talk to her about going to school and having others have to take care of her........
She'll catch on.....just hang in there.......I'm not sure the miralax is a good idea.....what is the Dr's reasoning on this? If she is holding, it doesn't matter what she takes or not............
In any event, you hang in there, take care, be positive and everything will get better.