I was sure there’s a way to get the Relative location of a Socket to it’s parent mesh… maybe not though.
What you can do is use InverseTransformLocation() to get the World Position in Relative Coordinates. It’s been a while since I’ve needed that function but IIRC it’s relatively easy to use (double check this syntax though)
VectorToTransform.InverseTransformLocation(const FTransform RootTransform);