SPECULATION How did Snoke know Ben had Skywalker Blood?

In TFA Novel, it is said by Leia that Snoke watched Ben Solo from the beginning, knowing he would be powerful in the Force and that he manipulated him from the shadows. It appears that Snoke knew all along that Ben Solo had Skywalker blood. That Leia is Luke's sister. Another child of Anakin Skywalker. But the question is, how did he know this?

The only people to have learned this were Bail Organa, Kenobi, Yoda, Luke, Han and of course Leia. Sidious knew of Luke but not Leia. And Vader only learned about Leia shortly before he died. (So that rules out the Sith ever passing the knowledge to Snoke in some way).

Now we can essentially rule out Bail, Yoda and Kenobi telling anyone else. So that means that Luke, Leia or Han must've told someone. It should be noted also that it doesn't appear to be public knowledge that Luke was Vader's son (In the novel Snoke talks about how the historians have it wrong about what happened, putting it down to strategy and arrogance when it was in fact sentiment - implying they are unaware of Vader's turn to the light).

What is also interesting is that both Snoke and Kylo know exactly what happened in the throne room in ROTJ. They know Vader saved Luke and killed Sidious. So either Luke told Kylo this and he told Snoke or Luke confided in someone else and this person informed Snoke. I'd imagine that Luke or Leia might have told Ben to try and keep him from the Dark Side, but even so...

So the question is, who passed on the information to Snoke that Leia was a Skywalker?
If Han, Luke or Leia told anyone they would know the risk - they would know that it would potentially put themselves and their kids in danger. And is there any reason for anyone else to know?

The only other person we are shown to have had a connection with Luke and Leia is Lor San Tekka. Kylo Ren's remarks about how old he looks suggests he has been around the Skywalker family a lot. But how would he fit into all this? Did Luke and Leia trust him enough to tell him this information? Does this connect with the reason why he has the map to Luke? But if he told Snoke, what would that mean?

Or did Snoke at some point befriend the Skywalker's himself and betray them? Did Luke or Leia share this information with him?

I just think this is an important question as it could tell us a lot about who Snoke is.
So, how did Snoke know that Ben Solo was the grandson of Anakin Skywalker?

 

But Snoke was watching Ben before this. Leia only sends Ben to Luke when it's going bad with him. Snoke knows of Ben's Skywalker blood from birth.

Leia explains that Snoke manipulated things from the shadows so we don't know how close he got. And Luke and Leia were known to be friends so if the were affectionate then that doesn't mean they are siblings to the causal passer by.

Vader picked up the fact that Leia was Luke's sister from Luke but only before Luke was a fully established Jedi.
It's not beyond the realms of possibility that Snoke just came across this knowledge himself somehow but it strikes me that this means he would've got it from Luke by means of the Force - and surely Luke is too powerful to have Snoke do that to him?

 

But how? Sidious was around for a long time and he didn't know about the kids.
There has to be a point within the year after ROTJ where Snoke learned that Leia was a Skywalker.
Either that or he is Plagueis and and Plagueis has some kind of inside knowledge of the Skywalkers because he created them.

 

I wonder if Snoke had been working behind the scenes helping the Rebels during the OT in some way, thus getting on side and being trusted by them?
This then allowed him to get close to the Skywalkers and betray them post-ROTJ?

 

That's not what the novel suggests. The novel tells us that Snoke new that Ben would be powerful. That he watched him from the beginning.

Potentially. The only time frame to really find out about Leia being Luke's sister is within a year of ROTJ.