It's not consistent because some people are still getting money under the old system, which was more flexible and designed to help them cope if there were genuine reasons that they couldn't work.
The new system, with Universal Credit, PIP etc is designed to be punitive wherever possible. People are judged well enough to work with conditions that before would have merited full Incapacity Benefit and High Rate DLA and when they can't manage they are sanctioned. The structure of disability payments has changed to limit the amount of time you can get payment if you are put in the Work Related group of ESA.
The latest ruse is to call people in early for PIP assessments and then stop the benefit months before the original end date set out when the claim was approved. This has just happened to my best friend, who should have had it until October but has lost it after an assessment in March. Her condition hasn't changed and won't, so now she has to appeal, otherwise she won't be able to run the tiny, cheap car she needs to get her to hospital appointments.
So, if you are disabled and actually getting the correct level of benefits for your condition, you can live in a quietly comfortable way- no big holidays or designer clothes, but you can eat and pay the bills with a little left over. But more and more people are being denied that money as rules are changed and abused and ignored and they are in poverty with no way of getting back out except appealing against wrong decisions, which takes up to a year or so for some and doesn't pay any baseline rate in the interim.
Benefits are still being changed over at the moment, so some people are still getting DLA (which doesn't have a constant rolling assessment criteria because it's actually properly designed for people with long term and permanent conditions) and the majority of people with children are not yet on UC, which is again very conditional and will certainly be a lower amount overall once everything is taken into account.
Oh, the extra bedroom thing, some councils have exempted disabled people from the LHA decrease and scraped up the money from elsewhere. This isn't going to be tenable much longer and your sister certainly shouldn't assume that she will never need to pay I'm afraid.