Sounds like your mum had your birth cert changed, she probably did it when she regsitered your sisters birth. You should ask the registrar what the rules/laws are about changes - in most places there is a time limit typically of a few months.
I assume you and your sister have different fathers and am guessing your mum was not married to your biological father, so yes, you need to search records under her maiden name and see if you can find birth entries.
I just can't believe an original birth certificate for you has not been kept anywhere. I would make an appointment with the register office (or equivalent) and go and speak to them face to face.
Really good luck to you with this, it must be so frustrating....
Recently had our own family mystery to solve and got nowhere until we went into the regsiter office (UK) who solved it in minutes!