Two Manchester teenaged stepbrothers are being credited with helping to catch a robber.  Victor Rosario and Jonathan Tavares witnessed a robbery at the CVS on Mammoth Road early Monday morning.  The teens followed the suspect down to Exit 5 in Derry.  All the while, staying on the phone with police.  At some point the alleged robber showed a knife and told the teens to leave.  That’s when police arrived and arrested Seth Skillin and Robbi O’Rourke.  Skillins had $900 hidden in his pants.