When you use "sana", it describes a property of "mia onklo", i.e. it expresses that the uncle was healthy while he came. If you use "sane", it describes "venas", i.e. you are saying that the action of his coming was healthy, which doesn't make any sense.
Here is a sentence where both forms make sense:
- "Mi venis sole por ne ĝeni ŝin." This means that the only reason why I came was so as not to disturb her. (Here "sole" actually describes "por ne ĝeni ĝin" and not "venas".)
- "Mi venis sola por ne ĝeni ŝin." This means that I came by myself so as not to distrub her.
Some people use "sole" when "sola" should be used (I myself did it for many years), but I think that this deviation from the norm cannot be recommended.