What is the difference between the meanings of words rarely and sometimes?

Rarely and sometimes they have almost similar meanings, both adverbs suggest an action that tends to repeat itself over time. These words can be used synonymously in speech. Still, you can find a small semantic difference. Rarely characterizes an action that has a certain repetition rate.
Sometimes the action can be repeated only a couple of times and has a fickle nature, it can be repeated the next time, or it may not happen.