There is Chamber of Chat which is okay. But Warner Bros. has now created Pottermore.
No, Avada Kedavra is not a known spell in the Harry Potter games until Goblet of Fire. And you are incapable to using the spell in any of the games afterwards.
There are many Harry Potter RPGS (virtual games) out there. All you need to do is preform a search.
No Harry Potter doesn't have any sibling's.
Yes, there are plenty. One such example are the Harry Potter games.
yes
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Harry Potter does not have any brothers or sisters.
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You cannot legally download any of the Harry Potter video games for free. To play Harry Potter and the Philosopher's Stone you must purchase the game for a gaming platform or PC.
Ebay or amazon sell Harry Potter stuff there is a lot on theose websites: www.eBay.com or if you are from the UK go on www.eBay.co.UK www.amazon.com