Because "sister" is the subject of the phrase "...has done this before", the correct pronoun is "who", since "whom" is used for the indirect object of a sentence.
You could test which phrase you're after by trying to substitute "she" for "who" or "her" for "whom". This would yield the potential results "SHE has done this before" (which is correct!) or "HER has done this before" (which is obviously wrong, and sounds like a funny caveman's way of speaking).
And you need to insert a comma before the phrase "who has done this before", so you get "...my sister, who has done this before.".