It entirely depends on their needs, which dictates their care plan, sibling assessments of whether on balance they should be placed together or apart, if assessments say they should be placed together searches for done for that, but if no adopters or foster placements can take them together then the care plan may have to change.
Its a very complicated set of factors determining that decision and it can be fluid with contingency plans built in
With family placements for example you might get a grandma saying, I can take Johnny but I cant take Sarah. So she is assessed to care for Johnny, but a sibling assessment might say they need to be placed together, so you have to decide on balance what is the best case scenario, for Johnny to be placed with family, have a permanent place with family outside of the care system and not be severed from his family by way of adoption, or is it better he is placed with his sister which is one of the longest relationships children have in their lives, their sibling relationship
On the other hand she might want to care for both, but the assessment finds she cant meet Johnny's particular needs but she might be able to meet Sarah's needs, so you then need to decide again what the best option is.