My heart goes out to you, Dmac! I am so deeply sorry this is happening! I must admit, I can't for the life of me imagine giving up my baby, especially if I had a loving mum/GM to take care of her so much of the time, as you have been doing. I can't imagine tearing her away from a GM who has been her primary caregiver either, a 6-month-old knows who their caregivers are. But I don't mean to judge your DD and soon-to-be XSIL. And if DD really can't be around GD, it may be best for the child to live w/ other family.
"...the only time I have a real problem is when the parents don't come home and don't inform me. Then, I stay up all night waiting for someone to show up and end up frustrated and angry with them."
It sounds as if neither parent is totally committed to their child, which is very sad. The only thing I don't understand is why they didn't simply ask you to have GD fulltime. Maybe that would be "too much" for you, but did they even ask? Is her dad going to be living w/ her and his parents? Is that why she's going there?
Regardless, hopefully, DD will, at least, ask for visitation. Then perhaps you'll be assured visitation, as well. For now, I agree w/ others about keeping in touch and asking for pix, etc. I trust you won't push though, they will need time to settle into their new routine and for GD to get used to them (she will, but it may take a little time). Phone, email, or write, but not often, at least not till you see what kind of response you get.
Also, I agree w/ MOnica that you need to let yourself weep - grieve even for what seems like a "lost relationship." But don't show that grief to your XSIL or his family. For them, be pleasant, supportive, and yes, practical about contact.
Please let us know how it all works out. For now, sending lots of hugs and xx.