I agree it is great to make up your own game - the key is that everybody gets to know the rules. If your family wants to play by a different set of rules that's fine. It is also good to talk about the idea that other families or groups might have different rules.
I think you do teach turn taking and being a good sport, just be realistic in knowing that probably a lot of younger kids aren't going to get it really fast and that when they play in groups it doesn't always work out. This is also a reason to be careful about how you correct your kid - avoid calling them a cheater, avoid harsh punishment because likely whatever you say about rule breaking they will say to somebody else later.
I think most kids can get the idea that some activities might work better with parents than with other five year olds.
