It can be a very painful time for teens or pre-teens while they compare their bodies to their classmates and the magazine covers. In answer to your question, however, no it is not true that you necessarily end up the way you start at age 12, despite what happened to your cousin. Some girls develop early, some later. Some girls start growing very early, but then never get much bigger. Some start late and then get real big.
Breasts come in all shapes and sizes.
Some boys may be very interested in breast size and some may not. Not all people are interested or attracted by the same things. You can see this yourself if you ask your girlfriends who they think is attractive. There may be a lot of different answers. One might think one boy is really great looking, and another might think he's not attractive at all.