could you make a reward-schedule thingy for the both of them?
and give them a golden star/sticker/flower/... for every day without accidents,
and then maybe a small treat if they have a whole week of no accidents.
young children like that usually respond very well to such reinforcements.
also, if she sees her sister getting more rewards than her, she might pay more attention
to her own bathroom behaviours
also, before anything... has she ever had periods of being completely dry for a day?
and around when did that change, maybe her little sis getting more attention for going potty
is what made her have "accidents".
if she however hasn't had any dry periods, no need to look into that, also know it's perfectly normal
and we (in the professional field) don't worry about it untill the child is 'bout 5 to 7 yrs old.