It's probably the hardest thing i've ever done, but when I got a letter like that from my family I just respected their religious (cultish) beliefs and moved on.
That's been about 5 years now, and I plan on calling them soon to see if they've changed their minds (kind of like JW's do at a "do-not-call" home).
It's hard to do, kind of like emotional suicide.....but it's the right thing to do.