SPOILERS and conspiracy theories.
• Ok, Hodge was there, so Jace never had a chance see a photo of the Circle so he did not recognize Valentine as his ‘dad’.
• Ok, Valentine never posed as Michael Wayland in public, so nobody recognized him as one of the most well-known criminals in the Shadowhunter world. (probably it was just a show for Jace only)
• Ok, nobody recognized the Morgenstern family ring even if Jace had it on his finger for years because they did not visit Alicanteto meet other shadowhunters. (and Maryse just did not pay attention to Valentine’s jewellery)
BUT Robert Goddamn Lightwood, you shady bastard. I mean WHAT THE ACTUAL HELL-
• your parabatai’s death is kind of a big deal.
• you know, it hurts and bleeds and the rune is gone
• I’m pretty sure you noticed
• and then TEN YEARS LATER a kid comes to your house, and claims to be your long lost blood brother’s son, and says he JUST LOST HIS DAD.
• I mean ten years.
• and you adopt him, no questions asked.
Conclusion: I think Robert did lot more than cheating on his wife. You know, like being on the Valentine’s side all along.