my DC dads ( my ex) cousins son.

I looked at his profile and there is a picture of his mum, I clicked on her and I'm sure I met her at my exP mum funeral 5 years ago. My exP is on her friends list. I don't think he knows my DC are related to him, second cousins?
Is it weird? I should tell him shouldn't I? He was so nice as well, typical!