i've just been dumped. for no valid reason. I asked his sister what to do and if he would ever tell me; is that an okay move to make?

Yes, that was an okay move to make. However, since he dumped you for no valid reason, he probably doesn't have the honor to give you a reason. It's unfair - it isn't right - but sometimes, people will never give closure. I - from personal experience - would suggest (easy to say, hard to do!) accepting that he will probably never give you closure. Why? Because when you wait for or demand closure from someone who refuses to give it, HE has the power. You give HIM the power to make you feel better or to keep wondering. That's no way to live. You have to make your peace within yourself, and Stop Pursuing Him. You have to take back your power. It's hard. It will hurt. But when you say, "Wow, that sucked. He left me for no valid reason, and I may never know why. But I choose to believe I am worthy of love. I choose to move forward, regardless of what he was thinking. I have the power to move forward - I don't need his excuses or permission." When you stop asking for his explanations, you take back control, you take back your strength to move on. It's hard, but you can do it. I took back my power. I believe in you - best of luck taking back yours! You can do this!

As humans we tend to search for reason in everything, even if there is sometime no reason. Sometimes people grow apart. If you're close with his family, its okay to ask, but try not to deeply involve them in the situation. Just remember that with every bad thing that happens, there will always be new adventures and new connections around the corner. Try not to dwell on the negative experiences! Grab life by the scruff of the neck and live your best life! You're going to achieve great things, and hopefully you won't even remember this sadness! Good luck!