The same way you write it. "Hi you may not remember me but we are related" My mom and dad are.... and add simple stuff about you and not to much personal until that relationship is strong, you never know.
Its very easy.facebook is for that just simply write her..Tell her who you are and add her and before you know it yous will be chatting,Dont feel your being cheesy your family and want to be family absolutely nothing wrong with that.That is a good thiing. Good Luck.. :]