The money was most likely pocketed by either the boyfriend or the mate … that much we can say with some credibility. Of course the niece can’t be ruled out. I note they BOTH denied anybody being invited over, before floating a moody story about an ‘opportunist’! This undermines her honesty to a certain extent.
There are numerous possibilities. Perhaps the mate gained possession of the backdoor keys at some point during his stay, (which may have been with or without the boyfriend’s knowledge/collusion), with the intent to turn the address over later that Sunday.
Believing that the house was unoccupied he may have returned and attempted to enter via the rear door but didn’t penetrate far into the address, as he was unexpectedly disturbed by your returning brother. He managed to make good his escape without stealing anything or coming to your brothers attention.
Believe it or not some burglars can have varying M.O’s – some will nick only cash and jewelry (stuff not instantly identifiable) where others will have kit like PS3’s etc. Some will conduct tidy searches, others will totally ransack rooms.
Tell niece and matey that you want the £60 to materialize in the next day or two unless they and the as yet unknown mate will be getting nicked.