The letter really doesn't surprise me. I'm a 3rd generation born-in that faded and eventually years later DA'd for my own sanity but never told the family. Got married to a non-JW at a secular location with the reception at a secular location. My sister didn't attend the wedding or reception and nor did grandparents. Their reasoning was that I was baptized to the WT a JW and was marrying a non-JW so they couldn't attend. But we really should socialize together. WTF!? Funny how my mother married my non-JW father and the grandparents attended that wedding. It is a sad thing that some JW's pride themselves in following the WT line when they deem it must be followed but can look the other way at other things (a call to say "I remember when you were born XX number of years ago" but this isn't a happy birthday call!). I find it amazing that they will support and give every indication of they will attend and support you but at the last minute, New Light TM appears and it changes. I wouldn't be surprised if New Light TM says not to go to the graduation. Sadly.
I assume the parents are older, set in their beliefs and ways. Attacking what they've followed and had ingrained in their minds for decades really won't solve anything. If anything it may drive a wedge deeper and reinforce their attacking points. JWs do emotional blackmail well (experienced countless times). A non-response or a a simple "love is unconditional for you, siblings, children and friends transcends religious beliefs. The familial bond should be unbreakable and there for all". But alas, wasn't there a recent WT article that the WT bond is more important than the familial one? I try not to keep up too much on which way WT thoughts blow this week vs last week. But I digress. Exemplify love through actions, they speak louder than words.