There are so many reasons grandkids might not get equal treatment. Living closer to one family, a child (or family) with greater needs, or conversely, even a child with an 'easier' personality whom the grandparent finds more rewarding to hang out with. I'm truly glad I don't have to juggle favoritism issues since I have only one grandson, but I can see how easily that could happen.
As GrammaRocks suggests, use as much as you can as a learning opportunity for your daughter – how a person's behavior can affect other people. Keep in mind, too, that if you 'expect' grandma to treat all her grandkids the same, you are begging trouble. I doubt that it would be humanly possible. Be careful not to lay out a sense of guilt entitlement to Grandma's time, attention or gifts; that will not only drive Grandma away from you, but can only result in disappointment.
Rather than wait for or hope for Grandma to choose the time and place for connecting with your daughter, it would probably be more fruitful to invite her for visits that will include treating her to lunch or dinner or some special event. It's possible that she feels that you're leaving too much of the "work" of these plans up to her, and perhaps feels a bit taken for granted.
It's possible, too, that her daughter already does this. Or that her relationship with her daughter leaves her feeling obligated to help more with the other grandchild. You can't see into that relationship well enough to know whether there's any real basis for comparison.
Two thoughts jump out at me from your request. One: I dearly hope you're not reporting back to your daughter the facebook posts that Grandma makes. Two: If she is actually, habitually, making promises that she doesn't or can't keep, talk to her about how disappointing this has become to your daughter. Ask her to keep her visits more spontaneous so you and your daughter won't be let down again and again.
Your concerns are not nitpicky, in my view. You rightfully want your daughter to enjoy the fullness of all family connections. It's possible, however, you don't have enough information to consider the whole situation your MIL is dealing with, resulting in a sense that you're coming out second.
She may, in her own view, be giving all her grandkids all she's got to give, depending on their needs. So, if your daughter's needs arent' being met, SHE might need a little more information to realize that. But I hope you'll be careful to sound grateful for her connection with your daughter, and not blaming about the ways she falls short.