hello
@Bernycol
welcome to DTP
personally, I too think that the amount (over £20,000 a year) is far too much if it is simply to cover a fair share of household expenses ... your mum will, presumably, be paying for any personal expenses herself ...it doesn't sound as though your mum needs hands on personal care (so wouldn't yet qualify for Attendance Allowance, so your sister won't yet qualify for carer's allowance) but it is worth checking this as often those living with the person have a very different experience of their situation than those who aren't there all day every day
have a calm chat with your sister just to ask how this figure was worked out and exactly what your sister pays out for your mum ... you may want to mention Deprivation of Assets, as Local Authorities are careful about how money has been spent once someone is likely to need residential care at some point
maybe also mention that any Attorney works under the understanding that the Attorney does not benefit financially form being an Attorney ... though, from what you say, it doesn't sound as though your mum is in any way considered to no longer have capacity to manage her affairs, so the LPA isn't in effect ... your mum, therefore, can make even (what others consider to be) bad choices, though your sister as her future Attorney ought to keep those responsibilities in mind
you might want to contact the OPG to ask for their thoughts ... I would, though, keep your enquiry as asking about a possible situation to check whether it would be acceptable/good practice ... the OPG doesn't like to have Attorneys arguing amongst themselves when issues could be sorted out between them, and can, in extreme cases, take the Power away from the Attorneys, which you don't want