Here are the steps I recommend, and many families have found seperately adopted sibs.
1) Register on Julie's Sibling Registry. You can find the detail here:
Karen's Adoption Links - Siblings Registry, Birth Family
2) Join the regional group and post an inquiry
Karen's Adoption Links - Regional Email Groups for Russia, Ukraine, Kazakhstan
3) Contact all the agencies who are or have worked in that region:
Karen's Adoption Links - Russian Region - Adoption Agency
4) Post on the all the big lists for Russia: Adoption forums, A-Parent-Russ; FRUA chat; Russian_Adoption
(You may need to check on each groups policies for this type post)
Do you know if the sib was placed in the US?
Also, if you'd like to PM me, I will forward the information to a few groups.