I know it is usually customary for the groom's parents to pay for the rehersal dinner. But I don't think they know that. How do I bring it up to them politely? Because they are talking that they don't have to be at the rehersal. Help me.

Can't you have your fiance bring it up to them? After all, they are his parents!

Yes, I think it is only appropriate that he bring this up to them. It was an awkward subject for me too, but he will know how to handle them better. If they refuse to pay for it, there is not much you can do besides try something cheaper, like an informal barbeque.

I agree, have your FH mention it to them...it might be awkward coming from someone else.