My sister in law took upon herself to be the driver to her doctors, and I thought this was great of her until I became the 24/7 caregiver ( I work from home). I decided that it would be best if I had a one on one with the doctor that recommended my mom drink ensure meal replacements. Her weight is back (berry,yogurt, protein, banana...smoothies and three square meals a day) and she has gained without that sugar product. The problem with that was she has thrown in the towel entirely because I wanted one visit alone. She has created an argument in front of my mother and now my mother hates her. I took her off a medication and her night terrors stop, she actually carries on conversations now. Why must pride be a factor in this? (doctor said pill could be stopped without withdraw issues (worth a try)). I am too friggin tired to deal with drama when Mom has improved. Maybe I should have put this in "whine". She has alzheimers. I am just venting.
Maybe SIL was looking for a way out of taking Mom to appointments and to make it look like your fault. Maybe it is about control or pride.
Whatever it is, you don't have time for it.
Of course, only you know the sibling, and many here have siblings to ignore for the sake of ourselves and our parents. Trust your own instincts and experiences.
Do you really have to report to SIL or brother what you did and did not do as far as doctor's advice? I report everything to my own detriment at times, wanting to share is normal. Since I am doing what is right, I could keep it to myself.
You can post here, like you did Jackboy. This is your post, if you just want to vent, that is okay too. I am used to having discussions by myself, but there are so many who can relate to you.
IMO, there's bruised ego involved. Not your SIL thinking you're actually wrong. You may be a very straightforward communicator who didn't give a second thought about how your report of talking to the doctor may have sent the message that she dropped the ball.
It may even be the way you FEEL. Orrrr, you could be so proud of yourself that you're doing a bit too much crowing. ;) (BTW, I'd be darned proud.)
At any rate, SOMETHING ruffled feathers. Figure it out and fix it would be my heartfelt advice.