Since they're your parents and it's very clear favoritism between your own children, I would definitely say something. I would go in determined not to get emotional about it or your parents will discredit your argument as being overly emotional. You should write down the main points you want to make and stick to those. You should also have some concrete examples in mind and stick to facts. Explain how it makes you feel and how your younger child feels.
I would NOT bring in your sister-in-law on this. If she has issues about the favoritism, it's up to her to bring it up and discuss it with your mother. Keep it about your kids and don't bring your niece and nephew into it. Don't say anything about how other people notice the same thing happening or anything like that. Stick to what you see and deal with and don't bring anyone else into the discussion.
Then have some resolutions ready. When the boys get gifts you expect them to be equal. When the grandparents spend time with the kids, you expect the time to be equal. If the grandparents compliment a child in front of the other children then compliments should be given to each. When there are discipline issues, you and your husband will handle it and your judgments on discipline are final and shouldn't be questioned especially in front of the kids.
I wouldn't worry so much about the Facebook thing, especially since you can delete comments when t hey seem one-sided if you think your children will see those comments and notice that one is getting more than the other. That's not usually something I personally notice, though.
In my family, my kids were the only grandchildren until this month until my brother had his first daughter. My parents are just tickled, and even still my parents are pretty good already about not showing favoritism. But I hope they shower that miracle baby with gifts. She deserves them.
On my IL's side, the favoritism goes to the 2 boys. There are 7 girls. My eldest daughter gets some special attention because my husband is the only surviving son, though. It's just how it is in Italian families sometimes and my IL's are quite elderly and set in their ways. They're also under a lot of stress since my FIL is now in a nursing home, so I have to cut them some slack.