He's maybe nervous of her reaction too? I would suggest he tells her as soon as, 'cos whatever her reaction, She'll be mad if someone else tells her.
Families are weird, weird things and the best of them go a bit daft over weddings. WW3 nearly broke out over my wedding (though we sorted it out) as my dad had some very set ideas about how my wedding would go which obviously were about a million miles away from what I wanted. We negotiated something that suited both.
I hope most people will be happy for you but you can't please eveyone. Have you talked to your H2B about this, I mean properly - tried to find out why he's delayed telling her and why his family are saying that he'll never remarry? Maybe the trauma of the first marriage has sort of put them all off going through it again? Seems daft but things are different for you two than with his previous marriage, right? The most important thing is that you two are happy and are getting married, everyone else is peripheral. Or at least they were for me.
Hope it works out ok and this ramble (i've had a couple of glasses of wine) makes sense!

Rachxx