Sun Yongtian and Deng Fuming - both 3rd generation Sun students? There is probably an answer somewhere there. The long time students know who the seniors are. (Even Ms. Sun knew who her dad's senior students were.) As well, I can't see a 'traditional' lineage (regardless of the brevity of the style) not having some lineage/succession in place to ensure the style continues.

I think the question asked is a legitimate one; even though knowing such things may get in the way of some people making money from their association with the Sun family or prevent them from enhancing their own status.