Is it possible to have one of your miis cheat?

It is possible for Miis to cheat if a confessor is good enough, when the confessed mii accepts, the previous mii that was sweethearts gets depressed

Hi My nickname is Farty. Yes, I guess ts possible for them to cheat. And even break up! Maybe, cause my boy mii already has a special someone and he was ignoring her and hanging out with another grl, if you want them to bond with eachother more TIP: if you want you're miis to stay together give one of them a travle ticket so they can spend time together,
TIP 2: put them in a group in the concert hall so they can spend time singing together and work in a team, press sing solo make group select miis and sing.
TIPS 3: when you're miis level up give then gifts and give them the same things so they can use the item together.
I think that's all, well, hope this helped! Cakes! From Farty

Yes they can. I try to level up my miis in sadness so I said yes when someone wanted to be with someone who was take. And he said YES :O

Yeah here's a story: once apon a time, I had a mii—ash ketchum—he was in love with misty, then sometime they broke up, then he dated dawn, they broke up. One day, ash was in love with bianca. Then they dated, and married. They had 5 children—leina (oldest), lily, lole, and lillian (youngest). The end.

No, they cannot cheat on others. I have tried it so many times and it has failed. Sorry if this isnt the answer you wanted