Do I refuse to go or go anyway?

Here's a little background: we get my step-daughter 1 week out of every month. Regardless of if she's here, we are never invited to any of my husband's family's events. My step-mother in law is a total bitch and uses my father in law. 
So today they wanted to come see my stepdaughter. Which is totally fine. However they wanted it on their watch and said they would text when they're on their way. They didn't even give us a "round about time"!! 
So they texted at 1:30 saying they were on their way but C (stepdaughter) was napping. All of the sudden it's inconvenient for them to come to our house so we then get invited to their family bbq tonight. 
It wasn't their first thought to invite us, only after their "time slot" for C didn't work did they invite us. They could have come when she was awake but it wasn't when they were "free."
I would never withhold C from them but I also won't allow them to only invite us when their original plan of not inviting us to the BBQ but still seeing her doesn't work. 
This is not the first time this has happened and everything just HAS to be on their schedule or it doesn't happen. 
So my question is: do I put my foot down, and refuse to be treated this way by not going? Or do I suck it up buttercup and go anyway?

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