Legally what should be done?

This is a sticky situation. My MIL is getting on in years. She still has all her marbles but she has some health issues. She has named my SIL as executor of her estate which none of us kids have a problem with. The problems lies with the SIL ideas about medical attention. She does not believe in doctors and prefers to self diagnose and doctor herself with herbs. that is fine if she chooses to do that for herself but there is some concern that if mom ever needed medical decisions made for her that the sister in law would not want her going to a doctor and would attempt to doctor her with herbs.this is a major problem as my MIL has had breast cancer and recently went in for her 1 year check up and they found another lump. they biopsied it immediately and when they did the lump went down. Right now we are assuming it was a cyst and that everything will be okay. Last year when mom was told she had breast cancer immediately the SIL wanted her to take a bunch of herbs rather than have the recommended lumpectomy and radiation.Recently SIL has came home and I was not happy at all with some of the things she was saying about major medical care and the hubby and I are more than a little concerned. Some examples of her ideas:

1) Everybody has cancer at some point in their body, it comes and goes so therefore you have lots of time to make decisions

2) mammograms are bs. you are more likely to get cancer from the mammogram than to have cancer without it. the chances of getting breast cancer are slim( but yet her mother has it)

3) if you have a broken bone just hold a cat and let it purr. the purring will help mend the broken bone.

4) all she can talk about is how herbs are beneficial. okay I have to admit we as a society have over used certain drugs and we are going to have to go old school but not for something like cancer or heart disease and then they should only be taken with your doctors approval

5) she doesn't believe in doctors and has the attitude that she knows more than they do

What the hubby and I would like to do is ask mom to allow him, myself and his twin brother to be her health care proxies and for her to make out a living will spelling out her wishes if she in an event could not make medical decisions for herself. Is this out of line? Is it a good idea or would it just stir the pot and cause hard feelings? If we don't get this done what kind of bs will we have to go through to make sure mom gets proper medical attention? what rights do we as her other kids have regarding this decision? Oh and the reason the hubby suggested I be on the health care proxy with his brother is because as he put it there are women issues he is not familar with and we are married and have been for a very long time.He values my opinion and input.He thinks the three of us could work together to make sure mom's wishes are carried out and she gets the health care required.