The polite thing to do, when you are a guest, is to be aware of the reaction of your hosts and follow their lead. If the parents seem unhappy with you sunbathing nude or topless, then don't do it. Even worse, if his father ogles at you, then definitely cover up! But the sister's reaction seems positive, so keep her as your ally and sunbathing companion - or go to the beach as another reply suggested.
But let's be honest, sunbathing topless has been around for a couple of generations now - I bet his mum is happy to do the same. I'm 55 and always sunbathed with as little as possible right up until my early forties. Now it would be a bit unsulting to the general public, but if I was at a private pool you can bet your bottom dollar I'd be stripping off - it's healthier - with lots of SPF, of course. In fact, at a recent holiday at a friend's villa, and screened by trees, we were all skinny dipping. As we were all in our fifties we were no match for the likes of you, but it was such fun!
So (apart from when there are oglers around) stop thinking of your naked body in a sexu4al way and use it as an opportunity to give your body as much Vitamin D as possible to keep you going in the winter!