how?

[Lexi Rodriguez]

how can our dad be rabastan lestrange to change his name into william lestrange. his not related to any lestrange in the book.

 

[Sophia Stark]

I believe Kaela asked answered your question over here:

Normally I'll tell you to contact Alicia, but since she's sick you can't.    I'd PM either Professor Nicolas King or Professor Cecily Rambolt.  Both of them are super nice and super helpful and woud be more than happy to help you.