I.e. Opus is the largest and best model of each family but Sonnet is the first model of the 3.5 family and can beat 3's Opus in most tasks. When 3.5 Opus is released it will again outpace the 3.5 Sonnet model of the same family universally (in terms of capability) but until then it's a comparison of two different families without a universal guarantee, just a strong lean towards the newer model.
Opus is the largest model, but of the Claude 3 family. Claude 3.5 is the newest family of models, with Sonnet being the middle sized 3.5 model - and also the only available one. Regardless, it's better than Opus (the largest Claude 3 one).
Presumably, a Claude 3.5 Opus will come out at some point, and should be even better - but maybe they've found that increasing the size for this model family just isn't cost effective. Or doesn't improve things that much. I'm unsure if they've said anything about it recently.