Hi Herb.
You're problem seems pretty serious.
There's a lot that you would need to look at to find what the problems are that are causing this. I would recommend seeking a professional that can visit with you in your home to see what happens, how, when, and what you do when it does occur.
There's no quick fix that I can give you that would fix the situation.
However, one common problem is that dog owners tend to label dog behavior with human behavior. Dogs are animals and need different things than humans so associating a human behavior such as "he's jealous" may only distract you from the real issues they're trying to resolve.
Is sounds to me in my opinion that there is a battle for rank between the two dogs. The other smaller dogs don't want to fight for position, they're fine in their rank so they stay out of it. The two dogs fighting may be looking to take the higher status in your family. So when your family members arrive they may feel they need to fight for position. Plus if the family expects a fight to break out it may also trigger an attack because the dogs can sense tension.
I recommend you consult a professional right away. I recommend
Glenda Herrin. At this point you need somebody that can explain their behavior and show you what you are doing wrong as a dog owner. It's not something you can solve with basic obedience training or something you can search the internet for. Each case is different so it's best to have somebody come to your place and get a better idea of what's going on.
You can visit her website at
www.heelinghounds.com and make an appointment since there is much more to look at than I can do on this message board.
Best of luck! Please let us know what happens!