DEAR AMY: I got married (at 30) to a man who had two children. The girl was seven, the boy was two. We were together for 16 years before divorcing, and although I have a close bond to the now 53-year-old former stepdaughter, I have never been successful in having a bond with the son. Now, 30 years after my divorce from his father, I received an invitation to his daughter’s high school graduation party. I am flummoxed and am unsure about how to respond to this gesture. I do not know this young woman and have never been included in their lives. Should I just send a nice card? – On the Fence DEAR…