So my brother went to pick up my payment summary from my former employer as he still works there and upon arrival he realised that it had already been opened. What can l do from here? And is this legal?
I agree with @winston wolf because there are plausible explanations that may justify the opening of a payment summary by an employer. For example, for your manager or payearoll department to confirm the accuracy of your payment summary.
As a sidenote - I know of many friends that work in offices where a central mail room opens most mail (including personal mail, such as birthday cards) before they reach the employee so unless you have evidence of something more sinister, it may be better to move on.