I believe ignorance breeds intolerance (or is it the other way around?) and the only way to combat it is by educating those who are ignorant. Do some research to see if you can find any stats on famous people who have occasion to crossdress. Then ask your in-laws if they think those people are "flamers". It seems to me that all they know is what they see on programs like Jerry Springer and his ilk. The people on those shows are usually the exception rather than the rule. If you can provide them with positive illustrations of everyday people who enjoy crossdressing, that might be a baby step in winning them over.