Its always tough to raise your prices once you have done something for free. Just like its tough to tighten the rules once you have set the norm at "carefree".
This type of situation stinks. The only good advice I can give is for everyone on here to set standard hourly rates that you can reference if anyone ever asks. Like it or not, if someone was expecting zero and you say $50, their going to be less than impressed. If you say you normally would charge $100 but because they are a friend you will only charge $50, they feel like you helped them out.
If you stop to add up the time, fuel costs, wear and tear and loss of your family time, it should cost something. There are even times I would charge family if they acted more like mooches in the past rather than kin.
Each case is different, but you have to draw the line somewhere. True friends will always be happy to at least offer something (more than warm beer). Friends who don't appreciate your efforts really should not fall in the "friends" category. As the saying goes, with friends like that, who needs enemies!