Hendrik Plug - Dutch immigrant Plug family of Western Australia

There's quite a few Plug's in Albany and Fremantle Cemeteries however i done know who is related to whom.
Could Hendrik Plug
be a cousin or a sibling of
Jan Plug
as they are married to sisters?
There are an extensive list of Plugs arriving on 17/8/1958 in Fremantle onboard the Sibajak. I suspect at least two siblings and their families. Jan and Hendrick being children when ariiving therefore are probably either siblings or cousins