Is it okay to video tape or take picture of wife and his relatives without their consent for gathering proof against them? Isn't privacy law come in to picture here? Also people submit e-mails as proof which family members wrote to each other before divorce petition. Isn't that against privacy law?
See taking pictures and exchanging emails are part of evidence, but the same must be taken casually so that both the purposes would be solved, but don't take odd pictures which would go against you only.
If your intention of taking video tape or pictures is not bad or evil it is ok. Since you are using the above videos and pictures for evidence purpose, it is not illegal. Here your intention is not bad. If you are using the above videos and picture to blackmail to extract some money etc is illegal.