Will sleeping with my son's wife have a negative effect on our relationship?

It's worth noting that when the child is born your son will mistakenly believe it is his and act accordingly.

However there is a

• 'The husband suspects something ain't right'
• 'On-action event: Pregnancy in matrimony, with another father!'
• 'The father's wife is angered' events.

So there is still plenty of ways this can go wrong and ruin your relationship with your son. If the child does become your bastard you can choose to legitimise him though. I would see how the new childs attributes compare to your own sons. Then work out which one to favour. You can always have someone locked up if necessary.