inmyshoos - wow, I wish I'd seen this post earlier, it looks like you could have done with some more support.
I've been where you are, I have ILs who favor one of their four sons, over the other three they have. They also favor the GC of their favored DS.
In our case, my DH and the other two non-favored sons lived NEARER to their parents, than the favored son. Yet more effort has always been made to visit and support the Golden Child and the Golden Grandchildren.
DD2 was born in June 2013, and it was SIX weeks before my ILs saw her for the first time, and then they only "popped in" on the way to visit the Golden Child, as the Golden Daughter-in-Law had given birth to their third child. Yes that's right, my ILs only popped in to see one newborn granddaughter (they stayed for less than an hour), as they were on the way to visit the other newborn granddaughter. They stayed there for the whole weekend.
And I have many stories, similar to yours, so I do understand how you feel and where you're coming from.
As a family, we moved to the United States in December (just after Christmas), and despite everything we were keen to set up regular contact between our DC and my ILs.
My FIL is very tech savvy, and already had Skype and that set up, to keep in regular contact with the Golden Family (who live three hours away from them). We left the UK with promises of regular Skype chats, which were arranged for a certain day and time, that worked in both countries.
The first Skype chat happened, and I said to DH afterwards, "You know what, I thought your parents would let the DC down, but they didn't. Maybe us moving so far away has actually triggered something in them." Alas I spoke to soon.
The second week came, my DC sat there ready, but nothing happened. DH said, "maybe they got held up, I won't say anything." Then the third week came around. Then the fourth. Then the fifth.
DH sent e-mails, left answer phone messages, and yet we heard nothing. Before long, THREE MONTHS had passed, and we had had no contact from my ILs other than that first Skype chat. So DH stopped trying to contact them.
Then I found out I had fallen pregnant again, and we decided not to tell them, as they clearly have no interest in our family. However, totally out the blue, we had an angry e-mail from his father demanding to know why we hadn't told them I was pregnant. It all went downhill after that.
Anyway, I'm aware I'm going on, so rather than go on and on further, I'll just say this...
We have now cut FIL and MIL out of our lives. It is the most glorious feeling, more for my DH than me, and he actually told me a couple of weeks ago that he feels a huge weight has been lifted. He has clung so desperately to the very poor relationship he had with his parents (through their doing, not his), in the hope that one day they might show the same kind of love to him (and his other two brothers), that they do to their favored son. I think all this has made him realize, this is never going to be forthcoming.
And no, our children won't suffer, in fact they were suffering more from clearly being made to feel less worthy than their "golden" cousins. They had noticed it, DS2 (8) had commented on it before.
Only you and DH can decide whether it's worth continuing to have his parents in your lives or not, but whatever you decide, please know there's someone here who understands how you feel.
(I don't normally do this, but if you ever want someone to talk to, feel free to message me via the personal message system).