I'm probably the least qualified person to post an answer on this type of question.... however,
I think when you love someone, like a member of your family, its the kind of love that is unconditional. You know them so well that even their irritating flaws are tolerable. You are comfy with them, they also accept your flaws. You also know that if anything happened to them you would be devastated, and feel as if a limb had been cut off. If you have a row and fall out, you know it's only a matter of time before things can be patched up.
Whereas, if you are in love with someone, you feel your whole being is dependant on them. They can dictate your happiness, they can also upset you more than anyone else in the world. If they withdraw their love you cannot function, mentally or physically, you feel that they have gone forever.
So like ummmm and Octavious say, you love your parents but do not feel the passion or need for them to love you in return.
In a new relationship, you can only be 'in love' but as it develops you are both 'in love' and love the person. However, when the lust, passion and dependency leave you 'just' love them, more because you care about them.
It ends up like a 'brother/ sister' relationship, a comfort zone has gone a little too far.
So, in a nutshell, ............... no sorry I can't define it for you.