If this abusive fil had any sense of repentance he himself would have acknowledged the past, apologised for it, stated his desire to move on (aka victim developing amnesia) and most importantly leaving the ball firmly in his son's court.
Step-MommyDearest would not be making demands or requests, this would come from dh at his own pace, in his own time.
This fil has not changed, he's just enlisted God to cover up for the sins he's perpetuated against his child.
He wants the script to be:
Fil: "I'm religious now"
Everyone else "oh that's all ok then, let's all have a cuppa tea"
He's only got religion to try and make himself feel better, Adopt a mask to further disable and disempower his victims.
"Ah, but he's your dad"
"He's an old man"
"He goes to church to be seen"
Lundy Bancroft in his book Why Does He Do That? States that the likelihood of an abuser reforming and being Not An Abuser is a fraction of a percent.
Abusers are weak and cowardly, they abuse because they are nothing and want to take the power from normal people.
He has only modified his behaviour, because he has no balls to take on an adult.
Op, tell your h that he needs more time before meeting them, and Christmas is an excuse well used by abusers to reel people back in.
It's ok to say he doesn't want to meet. I also worry that his "forgiveness" isn't forgiveness, it's fear that's driving him to suppress it.
Ask him how he'd feel if the meet up was cancelled, his immediate gut reaction? If it's relief, then the "invitation" is to be declined.
His father and his wife can think what they like, they've lost the right to be anything other than understanding