If you have a sister and she dies, do you stop saying you have one? Or are you always a sister, even when the other half of the equation is gone?

This quote is suggesting that even when someone dies, the bond between siblings remains. The quote is emphasizing that even though a sister may no longer be alive, the individual will still consider themselves as having a sister. The loss does not necessarily mean that the relationship has ended and instead it remains, possibly in a different form.