This is a very tricky question. Because as a professional designer you want to take advantage of any professional opportunity that comes in your way to increase your dossier. Plus you want to help your friends or relatives to have professional success through the elaboration of a good corporate image, fliers, banners, etc. There are indeed some questions that need to be answered before you accept this type of proposition, as indicated in  Graphicdefine.org.

Is this professional relationship going to improve your personal relationship?
This is the very first question you need to pose yourself, before taking the job or not. Remember, as a client, there might be a certain type of argument when the project starts developing.  You need to avoid professional issues from going personal, this might affect your relationship in the future.

What is going to be your compensation?
This is a very sensitive point to consider, because you can give out a little discount on your work or even you can give it out for free just like a favor. Remember, if you want to charge your friend or relative for your work, there’re most likely going to be complains. Just let them know that you’ve decided to give them a little discount or that you gave it out for free because of your relationship. Just be careful on your approach.

Who will your really be working for?
Maybe your best friend is a really easy-going person, but you never know what would happen when it all boils down to money and business. Your relative or friend will be the one approving your design at the end. Try to treat him/her like you would do with any other client.