You may be over reacting - the limiting of family seats is very, very common...even in fairly small schools. And how many seats did he get, anyway...that is the big question. If he was allowed 7 seats & gave them all to friends, I'd say be upset, but if he gave them to his choice of close relatives, so be it. I know one of my nephew high school graduation was held in a large auditorium, but his school was so big, they still only allowed 4 tickets per student, so any kid with divorced & remarried parents that's all they could invite. If a kid had 2 parents & 2 sets of grandparents, at least 2 grandparents were left out. My niece just graduated and her school only allowed 6 seats per student...her parents are divorced, so she has 3 parents/step parents, as well as 3 siblings & 4 grandparents & 2 great-grandparents...12 "close" relatives not counting all the aunts, uncles & cousins. But she could only have parents & siblings...no grandparents at all at the ceremony. Just have a big party afterward for all the extra relatives (and actually, we've found most of the relatives prefer going to the party, not the long, hot boring graduation.)
As for not putting in for extra seats, that sounds like a typical brainless teenager. And he probably thought he was doing the sisters a favor by not getting tickets for them.