I managed to figure this out. I had to display news in a scrolling news webpart. So I cloned the scrolling text webpart since scrolling news webpart inherits from scrolling text webpart. In my cloned scrolling text webpart I set the SiteName field type to multiple site selector instead of site selector which will have sitenames separated by a semi colon ( ; ) . and in the code behind I add this line of code below this line of code .

repItems.WhereCondition = WhereCondition;


String whereadd = "' OR SiteName ='";

if ( SiteName.IndexOf(";") != -1 )

{
repItems.SiteName = "##ALL##" ;
if( repItems.WhereCondition != "")
{
repItems.WhereCondition += " AND DocumentNodeID IN ( SELECT NodeID FROM View_CONTENT_news_Joined WHERE SiteName = '" + SiteName.ToString().Replace(";", whereadd) + "')";
}
else
{
repItems.WhereCondition = "DocumentNodeID IN ( SELECT NodeID FROM View_CONTENT_news_Joined WHERE SiteName = '" + SiteName.ToString().Replace(";", whereadd) + "')";
}

}

I then create a new scrolling news webpart which inherits from my the cloned scrolling text webpart. I now have a scrolling news webpart which selects news from multiple sites chosen using a multiple site selector