If you were financially dependent on the deceased you can apply to the court for proper provision.
If you think there was undue influence in making the will you can contest it - but you need hard evidence, not just 'there must have been' - and it will be expensive and still have a high chance of failure.
the only way it can have been unfair is if the sibling has been given something that wasn't the deceased's to give. there is no "unfair" when it comes to someone deciding what they want to leave people - it's up to them!