Is Petyr Baelish Robert Arryn's real father?

[Jaak]

16 hours ago, The Ned's Little Girl said:

It really isn't very thin. Looking at the chapter where LF pushes her out the Moon Door, she says:

 

 

I can't imagine why she would say "I would have given you a son too, but they murdered him" if she had actually given him a son in the form of SweetRobin.

I suppose it could be that SweetRobin is LF's son but Lysa didn't know it. If so, though, it's likely that nobody else in the story would know it either, which would make the whole question pretty much a moot point.

Yes. That´s pretty much the proof.

If Sweetrobin were Petyr´s, Lysa would have talked about the son she did give him, not the one she would have given him. If Lysa wasn´t sure whether Sweetrobin was Petyr´s or Jon´s, she would have told Petyr Sweetrobin was his. If Lysa was sure Sweetrobin was Jon´s, but thought Petyr might not be sure, she also would have told Petyr Sweetrobin was his.

Lysa´s omission to mention this proves Sweetrobin was not Petyr´s and could not possibly be, and Lysa knew Petyr knew that.