How to convert 17" bike to 18" without changing the frame
Without being able to get a larger frame, your brothers suggestions are about the best you'll be able to do.

Do you feel too compact on the bike (arms/hands feel close to your knees when pedaling)? 


pushing the seat back and longer stem will allow you to stretch out.
You can purchase a longer seat post if needed, they even make offset seat posts that would allow you to move the seat a little further back.


Do you feel like your are leaning over too much on the bike (handle bars seem too low and you need to crouch over to reach them)?


There are all sorts of stems in a wide variety of lengths available, even riser stems that raise the height of the handle bars so you are not leaning over as much.
You can also purchase riser handlebars that would provide some additional height to the front end of the bike.


I switched out from flat handlebars to riser handlebars on my mountain bike and felt much more comfortable. These new handlebars were also wider which have made me feel much more stable on the trails.