We all have secrets that only we know about. That's not what this is about at all. Family members are in most cases the first to notice changes in a person and can help to understand the problem before it grows from small to huge. That is why I believe that the closest people should know about your hobbies, at least about those related to risks and finances - trading, gambling. Of course, you should hide this information from underage children.
As I have said before the threat that comes with addictions is usually the tendency to also hide those from others, but while this is hardly possible with alcohol because you are wasted and can speak properly or move properly or other people can smell it, all of this is not a thing when someone suffers from a gambling addiction.
Even if that person would sell the car to have money to keep gambling, there could be dozens of excuses why that car was sold without admitting that there is an addiction involved. This is a very complicated issue and topic in general.