With three, I can relate...I have three grown ones. When they were small I was fearful that it would be hard for one to watch all busy bodies so we went out on very short nights....movie or dinner not both...and I hired two for two hours...
As they got older, one and paid my oldest (5 years older than daughter #2 and 7 yrs older than our son)1-quarter for every hour that she helped...this kept her/age 8 better behavied and very helpful to our sitter. Later, she babysat and the other two go paid a quarter for every hour if they were good....The eldest would not get paid if she was not a fair good babysitter....The result, everyone behavied or stuck together on their story that they were good....Also, expectations...handle it like a job interview.Ask your sister what she does with the kids...? You want someone who has had CPR too. Many kids who are in scouts have gotten their badges and CPR for babysitting. Planning is key and let your children know what they are expected to do and what a babysitter is suppose to do. This way if the babysitter is not good...they will tell you after she/he leaves. You want a fun safe time for all.
Also: think of every emergency scenerio and type it out and read through it with your sitter. See if they know what to do...put it on the refridge....I taught Girl Scouts for 12 years and I can attest, some GS are great babysitters due to their trainning...good luck. D.