You can add siblings and spouses to the Focus Group. To do it, first add the ancestors. Then, click the Select All button to select everyone in the FG. Next, under the Add Others section, check Descendants, and set it for one generation. Make sure no other boxes are checked, and click the Add Others button. You have just added the children of all the ancestors, in other words the siblings of their children who are in the direct line.
Next, click Select All again, and set the Add Others section to add only Spouses. Click the Add Others button again. You have now added the spouses of everyone already in the FG. Since you already had the spouses who were ancestors, you were really adding the spouses of the siblings (along with any "other" spouses of the ancestors.