My advice above was pretty chaotic.
If your parents have significant assets, the work is likely to be spread unevenly, or there might be dispute amongst siblings, it is worth using a lawyer for an LPA. You can then agree process, and charging and the latter acan be added to an LPA as a condition. I insisted on the ability to charge, not really for the money itself, but because I thought I would feel resentful if I was doing the bulk of the work and was also out of pocket. Sitting down in advance and agreeing, with your parents, how decisions will be made could save real battles and misunderstandings in the future.
POA is an administrative function. Your parents should not worry about their decision appearing to favour one sibling over another, but should instead consider who is best qualified to take it on. Proximity, skills, commitments etc. Much better for one person to be lead, with roles for others agreed. The sister living abroad can come over to help provide respite breaks, or help with a house move, but probably is not the right person for the day to day admin.
Checks and balances can be agreed in advance. For example I have offered my brother read access on my mums internet banking and a print out of her Hargreaves Lansdown account. He has not taken me up on it - perhaps because this is sufficient to reassure him that all is transparent. An alternative would be to have the accountant who prepares the tax return to prepare an annual Attorney statement.
Make sure there is an alternate for if the Attorney is incapacitated.
Again if there are significant or complicated assets, you might make provision for the Attorney to get on going advice. I probably need some as decisions I make will not only affect the inheritance of others, but could have tax or criminal (scarey, but even something small like making a Christmas gift to a really good carer is potentially a criminal act) implications. However without this in the LPA document I have to fund it myself.
Also before it is needed, try to get your parents to list their assets incomes and bank accounts, and where they keep key stuff like passports and driving licenses. Ideally they will start allowing you to "help" with their tax returns. Better still they agree to use an (your) accountant as their agent, so if something sudden happened the accountant can speak to HMRC even if LPA documents are not through.
Registering LPAs is a pain. The only solution I have found is to go into a larger bank branch with the original document, your passport, driving licence and a recent utility bill, get them to make their own copy of the LPA (so no need to pay a solicitor to certify - and they tend not to like own certifications) and let you fill out forms there and then, which they can then check. Then take the name and a phone number of the person you spoke to, as it is never straightforward.
I have just been Googling registering an LPA with HMRC, as I found a whole lot of tax letters in my mums flat. She happily says she is ignoring them as she doesn't mind if they send her to prison...I cant find any advice other than a paper designed for HMRC staff, and even that is unintelligible. Bulletin boards suggest that this is a general experience. The advice again is to take the original in by hand to the local office and have them photocopy. Things, apparently have been known to get lost. My task for this morning.