There are a number of ways of looking at this.
1. Go informal. Do as suggested.
2. Go formal.
In my experience, option 1 works really well in families where there is unlikely to be any fall out down the road. Although you can't always tell who is going to turn nasty, previous fallouts are a really good indicator.
Option 2 is failsafe. In the absence of an Enduring Power of Attorney ( and I think it's safe to assume that there isn't one) , have your mother declared a Ward of Court and the funds can be used for her welfare. This process can be distressing and expensive (medical and legal expenses) but you can rest easy in the knowledge that no-one can ever accuse you of acting in an underhand way.
mf