As painful and worrying as it is, I think your mum is right, it's non of your business. She is her own person and there are many reasons why people are unfaithful. It's really between her and your dad, you are your sister shouldn't be involved. And you really shouldn't be hacking into her phone, that's very intrusive.

You are 21 now, I wonder why your lives are so wrapped around your parents lives? I hope you find peace, but it's really up to your mum and dad what they decide to do.