Surely, if it's the mother's house, only she should have a say on who comes through the front door and who not.
I am curious though. How long is it since you've been inside the property and seen the interior? By which I mean does it appear that your bro and s.i.l. have 'taken over' the house, changed the decor, sold her personal effects and so forth.
At 90, with a downslope approach to the front door, there must be thoughts of moving her to another property, or a care home where there are no hazardous slopes to negotiate. Either way, that would involve sale of the house we're talking about and that leaves your bro and s.i.l. without a home.
No wonder there are tensions.