Hi. Go physically visit them for a good week to put strengh in those familly bond.
Remember that many do not like to type (not being able to type fast). Even if you just want a quick TY, they might already have a mind association email=unpleasent.
You could get them by the curiosity. Send them a zipped or rar or pdf text file with a password, with them needing to ask for the password ***** see it, or see the last part of a bit of news.
As for the time it take you, you could put them on a maling list and send then them all the same news. So it would not be longer to send it to one than to 1000 persons.
Put some interactivity in the news, contest, trivia etc... with the top score mentioned in the next update. Some with big ego will not want to miss recognition from the lowest class :)
Perhaps they are prefering other things than email, like facebook and would be more willing to participate that way.
Perhaps they just don't like frequent communication and would participate more if more time pass between them. Some may not want to respond because they feel sorry for themself to have an empty life with noting of interrest to report.