Capacity is a fairly fluid thing. As long as you are able to weigh up and understand the consequences, and make a decision in the best interest of the donor, you will be fine. If at any time, you feel unable, you can step back and let your sister take over for the period you feel unable to act for. The OPG has helpful info on their website, and I found this for NI. https://www.alzheimers.org.uk/get-support/legal-financial/who-attorney-northern-ireland
As long as you do the preparations with your mother, making the right decisions is not too hard. Talk to her about what she wants, how she wants to be cared for, who will care for her, what circumstances will change that etc etc.
I am attorney for my remaining parent who has been given a life limiting diagnosis. I share (J&S) PoA with my sibling. My sibling lives abroad, so day to day stuff falls to me but I keep them aware of what is happening. When there is large stuff or capital expenditure, I discuss it with them.
I care for my parent in their home. This is what they want. We have discussed that if the illness places them or me in physical danger, or if I can no longer cope with the help of carers, then we will be looking at residential nursing care. With regard to finances, I just keep everything ticking along, bills paid, house maintained, etc and if my parent needs anything, I facilitate that.
I also keep annual accounts. I've been attorney for other, now deceased, family members. As long as you have 'the talk' with your mother, you'll be fine, especially as you have your sibling for support.