Should I tell her or just mind my own business?

You're father-in-law must be a DILF even with his facial deformity.

honestly? the odds are pretty high. please remmber that the kids looking alike is pretty unlikely in general even if he WAS the father to both, so basically your question is :" what are the odds some kid would have the same facial deformity as my husband "(since your kid has it due to your hubby, so they aren't uncorrelated events). Hardly evidence of infidelity if you ask me.

It’s difficult I’d just as to see if they were a thing back n the day

I'm feeling like you're just looking for trouble.

Springer? Maury?

"Defect"? Wow, you must be a real class act.

We need more info for this to be a FML. A big question is: how long have you been married? The chance he has another child is one thing, him having another child because he cheated on you is world's different.