i think my uncle is cheating on my aunt

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So my aunt and uncle have been going through a rough patch the past few years, and she knows he cheating on her, but they're trying to making it work one last time. Well i was covering for my aunt at work while she was out of town for the week. Well last night i found something i don't know if i should have found, i found a not in my uncle old cd case. The note said " hi baby thanks for putting up with me even when an ill. i promise i'll be in a better mood tommorrow. i feel better just thinking about tommorow and tommorow night, i love you, i love you forever love(not my aunt). I was shocked, i don't know how old this note is. i mean this could have been before they got together, but doesn't that sound like an affair? I don't know wat to do, well my mom thinks he is an pig and i should tell my aunt, is it my none of my business?? or is it my responsibility to tell her?? help please