Maintaining Contact

Last night we attended a Christmas party for the local foster parents and kids. They had a meal and gifts for all the kids. It was a nice event.
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It gave the kids an opportunity to connect with their siblings. Our two foster babies have two older siblings, they are together in a different home. Their 5 year old sister spent most of the evening at our table playing with her sisters. Her brother was grounded to his chair but he did get to come over and say hello to them.
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My two, Anthony and Larissa, have two older brothers (different dad and case). One is a foster child in our county and about to be adopted by his foster parents. He came over and played with the kids. Their other sibling is 17 and does not want to be adopted. He is out of town in a residential home. We talked the the SW so we could go out and take him some Christmas gifts and let Anthony and Larissa see him.
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Our two also have a younger sister with serious medical issues. We would love to adopt her but the current foster parent wants to adopt so we are out of the picture. We were living in Texas when she was born.
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All those siblings separated. In a perfect world they would not live in separation. However, sometimes separation is necessary for the kids to heal. Sometimes it is a logistical thing. We can only take 2 at a time because in our state you are only allowed to have 5 kids under 18 years old in the foster home.
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Either way, it happens. The question is how do you deal with it? Do you maintain contact? How much contact? Is it even safe?
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I know that with the oldest sibling of my children, gang activity has been a big problem. In a residential home setting going for a visit is fine. What about when he gets out? When he gets out he will go to his dads home. Do I want him to know where we live? I really don't feel threatened by the bios of my two but I don't think I want to just give them the information either.
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Honestly, I don't know the answers. I think that this will be one of those things that we wing it and pray that we get it right because I don't think that there are any right answers out there.
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Well, nap time is coming to an end so I am off to change diapers.......