How do I politely decline building stuff for relatives/friends?

Easy, say yes but that you will not have time until September next year.

It's the same as having a hairdresser, dentist or a doctor in the family. Everybody wants a piece of their time at birthday parties to get a free consult, being the family geek is no different.

Only misery can come out of it because at 'free' the feature list is infinite and I don't like charging my relatives.

So I give them the address of a friend that builds websites for a living and they get a discount large enough that if it's serious they'll go for it.