It's easier to say, "My sister's friend is an actress."
How many sisters do you have? Does it matter? How many friends does she have? Which friend?
You could always say, "My sister Betty's friend is an actress." Or, "My sister Betty's friend Sarah is an actress."
Most anything is fine with me, except for the common mistake of using I instead of me. "My sister went to the play with Betty and I." That makes me cringe.