The answer is in the question.
Peter is not considered to be a Senior Royal or a Minor Royal because he is not a Royal.
The differences between Senior and Minor/Junior Royals tend to depend on relationship to the monarch, position in the line of succession, and duties performed. People considered to be Senior Royals are the monarch's children and/or the direct line, while Minor Royals are the grandchildren and cousins of the monarch. Junior Royals tends to be another term for Minor Royals, but I think the term tends to be used more for younger royals instead of older ones.
The Queen, DoE, Charles, and Camilla are always considered to be Senior. Anne, Andrew, Edward, Sophie, William, Catherine, and Harry are all sometimes considered to be Senior and sometimes considered to be Minor. The Kents, Gloucesters, York sisters, and George are all Minor.
William, Catherine, Harry, Beatrice, Eugenie, and George all also seem to sometimes get called Junior, in my opinion because of their younger age and lesser duties.