Keeping in touch with siblings after adoption?

Hello, We adopted our foster kids recently and they have several siblings in different homes in different states. We know of one family that has one of their siblings, but the rest we're not sure how to locate. Our kids are pretty young, but they know they have half siblings. They've never met them. Is there a way to find their other siblings? We have names, but that's all. No contact info. As an adoptee, I think it's so important for them to have a relationship. Also, I need advice about building a relationship with one of their relatives who has adopted one of their sibling. I feel strange and out of place. We recently connected and would like to build a relationship. How do I start? Where do i start? I feel clueless. How much can i share with her? I could really use some advice. Thanks so much!