My mother-in-law refuses to see the doctor. What can we do to convince her to go?

My husband occasionally stays with his 86 y.o. mother while he is doing work on her house and yard. Last week one evening, as they were watching TV, my husband said that his mother had a type of episode that looked like a seizure that lasted about 5 minutes, then she was fine. We know she is dehydrated and could use some fluids - electrolytes, as she does not drink much, and when she is outside (in the heat) she tends to overdo it and complains that she is dizzy and confused. We try to expialn to her the importance of keeping hydrated, especially in this hot, humid South Carolina climate! She has in the past fainted from heat exhaustion, but refuses to see the doctor. My husband called her doctor the next day and made an appointment for her to be seen, but didn't tell his mother until the day before the appointment. She was so angry and told my husband to leave her house and she wasn't going to see the doctor because she felt fine. His sister who lives 2.5 hours away told my husband that mom was fine and to go home!
I find it frustrating because we are trying to ensure that mom has the quality of life she deserves, but she is showing signs of dementia, dehydration and self neglect! We were the only ones willing to move down here to look after mom and everyone else has a life! We are treated like we are the enemy. Mom is very distrustful and thinks we want to put her into a nursing home. We explained to her many times that this is not the case - we are only concerned with her quality of life, safety and wellbeing. She lives in a big house at the end of a dead end private road in the country. She could have herself a nice little cottage or condo where she wouldn't have to worry about yard work or house repairs, or being alone out there in the woods! Since her husband passed away last September - at home - she is constantly reminded of this and complains that she doesn't like her house, but she would never move. She is very determined to do things her way - which is good in some respects, but it worries us to think she is alone and if something should happen, she would have no way to reach us. We've talked about getting her set up with one of those alert necklaces in case of an emergency, but nobody is taking any action to get it going. I think they all have their heads in the mud and in major denial.
I'm a CMA and have worked with elderly and their families. It is very difficult for the adult child to take responsibility for an elder parent. Mom wants her independence, but as far as I'm concerned, she is neglecting her health and she needs help! My husband has DPOA for health & finance, but has been putting off getting the documents filed with the court house, and giving a copy to her physician. All I can do is pray and give the burden to God.