Etiquette: is it wrong to mow someone else's backyard's lawn?

So my neighbors went on vacation and they asked some neighbor to mow their lawn and if he failed to do it then I was their second option... Well, that neighbor has failed to mow their lawn during the last 3 weeks or so .... therefore 2 days ago I decided to mow their front yard... However, I don't know if... show more So my neighbors went on vacation and they asked some neighbor to mow their lawn and if he failed to do it then I was their second option... Well, that neighbor has failed to mow their lawn during the last 3 weeks or so .... therefore 2 days ago I decided to mow their front yard... However, I don't know if mowing their backyard's lawn would be invasion of their privacy?
You see, I lack social skills... I guess i'm kind of autistic so I don't know which behaviors are acceptable and which ones aren't... According to my aunt i shouldn't cut their backyard's lawn because that would be intrusion of their private property.. she also argues that when the neighbors come back , if they find something missing inside their house, they will blame me. . but i find hard to believe that they don't have a camera or an alarm installed. .She says that mowing the front yard is alright because people can see me while I'm doing it..




It's just that I like mowing lawns... I'm not charging them money nor anything like that.

Thank you for your assistance.
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