In this scripture, Isaiah is explaining what a proper fast should entail. Like "mojo" said, Isaiah is pointing out Israels hypocrisy. "Your own flesh and blood" in this instance is your kinsfolk, your family, your siblings, parents, children, cousins, uncles, aunts, relatives, etc. What he is saying here is that you should not "turn away" or refuse to visit your family, especially if they are in need of your help. If you know that your family members are in need, you should go and administer to them, whether physical, emotional, or spiritual.
Fasting is a time where one abstains from the things of this world (food, drink, entertainment, ect.) to more fully dedicate oneself to God. If during this time of fasting you should refuse to help those that you know are in need, you are being a hypocrite. For how can God accept your sacrifice, when your heart is not right?