i would use a cms like typo3 or joomla. once you have learned how to use them it's much less work to do a website with them than doing one yourself from scratch with html and php. there are many extensions for different photo galleries and your brother-in-law will be able to log into the backend for making updates and changes in an easy way.

i like typo3 a lot and it didn't take me longer than a week to learn it. currently i am also looking into joomla which is smaller and simpler. probably it's best to get some book about them.