I didnt have time to read the other responses but did read your updates. I think you are being more generous than is called for. I certainly would not let my kids spend time at the inlaws without me being present at ALL times. Most likely I would make them come to us.
With the previous behavior I wouldnt put it past them to do something physically, emotionally, mentally to those children then pin it on you. It may sound far fetched but lets face it-- so is what they have already done.
YOU are the parent. YOU can decide who sees your children and when and what rules will apply. You have NO obligation to send your children to grandparents. They are adults. If they want to see them they know how to travel.
I would seriously rethink leaving them with in laws. No one says you cant change your mind. Just flat tell them that you have rethought it and in the best interest of your children they will have to come to you.
Also- although you do not have to give details do not be afraid to be straight with your children. Tell them that at this point in time grandma and auntie arent makeing wise choices and one of the consequences is that they must visit only at your house with you there. Simple but very rememberalbe lesson for children to see grandma have a consequence!
Feel free to contact me if you need support. From the little I saw of the responses you have lots!