POA and influence
Yes, someone with a POA can encourage some one to make a new will leaving him as a primary beneficiary; however, if the person with the POA is in a position of influence over the person making the will, it may be possible to have the will voided because of undue influence. This is a very technical legal matter and probably only an attorney specializing in wills and estate planning in your state, who knew the facts on both sides, could properly evaluate the situation.
If the person has a general POA, he could transfer property into his and his brothers’ names. However, if there was undue influence or fraud, the transfers could be defeated. Again, a lawyer would have to know all the facts to make a determination in this.