Anything slim is thin or narrow. So, your sister may be considered slim, and you could also have "a slim chance" of winning the lottery.
Slim people are slender, and slim things are usually delicate, like a slim watch band. You can also use slim to describe a margin or a chance: "There was only a slim likelihood that the home team would win this game." Back in the early 1800s, to slim meant "to do one's work carelessly," and in the seventeenth century, the adjective was also used to mean "sly or crafty."