Hang in there!
Doreen,
That is quite a range. My wife has a sister 10 years younger.
Remember it is worth all the investment you put into them. While the 14 year old has needs (my youngest is 14) it will benefit her to teach her how to properly treat her little sibling. I also believe it will benefit your 5 year old to teach him how to obey your 14 year old! To make that work you will have to have clear expectations on your 14 year old letting her know that she will be punished for abusing her "power".
God bless you in your important job of parenting!