Your brother needs to be loving but firm husband. She needs to get help if she has PDD which means she should go to a counsler or doctor, maybe couples counsling to help both understand how to handle their skills/situation better. It is very hard to switch someones mind from the state your SIL might be in, that is why professional help might be the best.
As far as doing stuff that she should not, no one can control her, she has to repsect herself to not do those things until fully healed. If you are near by offer to help, or encourage her to ask for help from others that do live near by. She does have a lot to handle, help is always appericated.
If your brother is really having issues with this he needs to seek help if unable to handle this stress. He may find that the best thing to do is not enable his wife, and a counsler might beable to give him the steps/skills to help his wife make better choices. Is she on meds, is this her just saying it and she has never seen a doctor? I would encourage her to go to a doctor, with hubby to work through this.
Edited: I am unsure if you mean PDD or PPD, two different things... either way BOTH I would still recommend her talking it through with a doctor.