A: A dream like that can be very frightening! I'm glad you sought out answers to its meaning. It doesn't always have to be taken literally, unless there is something in your waking life that would lead you to think that she is unsafe around water.
I would look at the dream more metaphorically, as a sign that you feel you're unable to provide help or assistance to this family member to the full extent that you would like to.
Bare with me for this last part, it's somewhat strange: often times the other characters in a dream really represent the different aspects of your own personality. This would be especially true if the family member is a descendant of yours. Think about what aspects of yourself this person represents. Are they aspects of yourself you wish you could "drown"? Alternatively, water is a symbol of the psyche, so perhaps these are qualities you wish you could internalize and make them part of you?
Hope this helps! Feel free to post follow-up questions.