Cloud Functions for Firebase: How to get event.data.ref.parent.child("xxx").val?

I'm having a structure on Firebase like this:

members
 |- -kSjJSDFjl2DFc
          |- isDeleted: true
          |- gender: "male"

In my cloud function, I wrote:

exports.doMemberDeletion = functions.database.ref("/members/{memberId}/isDeleted").onWrite(event => {
    const isDeleted = event.data.val();

    if (isDeleted) {
        const gender = event.data.ref.parent.child("gender").val; // Error here
        console.log("Deleted a " + gender + " member");
    }
});

However, it gives error on event.data.ref.parent.child().val, please advise how to get the value in this case. Thanks!