Tithing was for old testament Israel and is not an ordinance for the Christian church. Free will giving is taught in the new testament. However, this does not pay off as well for some pastors, so they mix a little "curse" fear and require the tithe from their sheep. Christians that have no idea of the Old vs. New Covennant fall prey to the tithing teaching. There is always some promise of reward assosiated with the call for tithing--and offerings.
Ask your relatives about the pastor's lifestyle as compared to theirs, they will probably tell you that he "deserves it". He probably has no money problems like they. The new testament says you are to give out of what you have not of what you don't have, whether it be 30% or 3%.
This guy has some of the best insight on this and debunks these "profits":
http://www.realanswers.net/realanswersaudio.html
Have your relatives listen to this audio on the Internet and his nationwide call in program. Many call asking about the tithe that is being required from them.