You can't do it without sounding like you think he's a paedo.
This man has known your dad since she was born, has always been her stepfather, and presumably always will be. Sharing a bed with him and her mother (her mother...) is just like her sharing a bed with you and your new partner. They are her other set of parents, and always have been. Get over yourself.
You are being alarmist and ridiculous, and enormously offensive. If you really think that your wife's husband of 12 years is a danger to your child, then whether she gets into bed with him or not is not the issue. You have a much bigger problem with contact at all.
It is your own mind that is seeing 'bed' as the problem. If you were not worried at all prior to your wife mentioning that she had slept with them, then you are just being offensive and it is your own mind that is drawing conclusions. If he was a risk to your daughter, the bed wouldn't be the only thing that rang alarm bells, particularly as she shares a bed with you, and at 5 years old, it would be perfectly natural for her to get into bed with her other set of parents as well.
Her other set of parents.
Her other set of parents.
I suspect this is more territorial on your part, and you are uncomfortable with the realisation that she gains support and comfort from the other relationship too, rather any stupidity about abuse.
You would rather see a 5 year old unable to sleep than have anyone else be able to comfort her in the way you do? Nice.