We're just in the process of sorting out LPA's (financial one and health one) for my mum (should have done at leat 10 years earlier - but she at least had the earlier EPA after her own experience in dealing with a old relative).
I would suggest that is a very important thing to sort before you go. (And their wills if they haven't already).
The LPA's (if it's what they want), should be for you and your sister to be "jointly and seperately" in charge of.
It can be done via the govt website - it is ALSO a lot easier to do it (signatures) before you go.
I would also suggest you get your will/s sorted out, and maybe start thinking about the LPA's for yourself - especially with respect to who would act on your behalf - we recently changed our will's because we had used people of our own age... and time moves on :-(