SIL is such a bitter person. She loves to get my goat. Will tell me my mother has food restrictions, but will take those same items to mom with a pic on facebook. Or has told me the pin number changes at the front door every month. Each time I have asked the professionals and they don't know where she heard that info. Claims she called everyone from ER, but my phone doesn't ring. I had told mom that I would bring x item next time I visit so she makes sure to say that it's a restricted item or change moms mind before I visit the next few days. Mom is still after almost 4 weeks waiting to get her phone in her NR room so now she isn't sure mom wants one. Even though she always chatted during TV shows every day. HELP??
And what was the reason that your Mom was placed in a care facility? The more information we have here the better it is for making suggestions to help you.
My Dad's memory was starting to slip a bit, and he was no longer using a calendar as every day was Ground Hog day. I even bought Dad a "day" clock which would show Dad what day it was, it was helpful for awhile. Dad still got the daily paper so he had some idea what day and date it was, but after awhile Dad would just go through the motions of reading.
As for the telephone, maybe Mom no longer can figure out how to use it, or she does use it, she is calling people she doesn't know. I remember with my Dad, finding cards sitting out from his Rolodex and thinking was he calling these people? He would have no need to call the plumber as he was now living in a senior facility. So one day I took out the cards that were not those of relatives.
I have no idea why your SIL has this need to control and keep you at arms length as much as possible, unfortunately people with this tendency usually show those colors at times like this. Maybe it's a need to be be important or her way of feeling self worth, it's sad for her a PIA for those around her and not worth expending the energy trying to figure out at this moment. You want access and care for your mom so as her daughter and as a MPOA you go ahead and insert yourself in that. If I were you I would direct important communication through bro, in fact make sure both (all?) brothers/siblings are getting updated info by sending it out to them when you get it from staff, take this burden off your brother (her husband) and simply assume each of your siblings will update their spouses and children. This might force SIL to deffer to you a little and it might not, I'm unclear as to what her husbands perspective is in all of this but if I were you I wouldn't force that conversation between the two of you unless absolutely necessary, let him come to you if his wife is complaining too much, again we are talking about YOUR mother and the siblings should come first, their spouses in support of the siblings who are supporting Mom. All the care your SIL has been and is doing out of love for your mom is supporting her family not as a decision maker, she is a decision maker in her family for her parents and in her's and your brother's family for their household, the decision power/responsibility she has taken on for your mom's household is extended her through her husband and his siblings, they still come first.
That doesn't minimize all she has done, no doubt she has saved you some 4hr round trips so please even though it's distasteful keep being generous about giving her that. There may be things going on, reasons for some of this you aren't getting and I wouldn't hesitate to just ask your brother who has been acting as MPOA those questions, even if they are ones you asked SIL and got odd answers to, if he says SIL said she already told you this just say, it wasn't making sense to me so I was hoping you could explain it better. Don't get the food restrictions from SIL, get them from NH, doctors, whenever is placing them and then feed her whatever you want. Bring the special items just don't tell SIL you will be or include her in that loop at all, don't give her the power to mess with you. It would be nice if BRO & SIL followed through with a phone but nothing prevents you from just doing it, maybe they have simply been too busy putting out other fires and trying to attend to their own lives as well, you don't need their "permission" to set mom up with a calendar in her room or an electronic one that shows day and date in big bold characters, a dry erase board or a simple planner whatever seems right. GL