I fancy his brother

So I've been with my fiancé just on two years and I obviously love him alot. But over the past 3/4 months I have realised I have started having mildly strong crush on his older brother. I mean my SO is so much more stereotypically hotter than his brother. Could it be hormones or instinct as his bother is heavier built, has a stable income and has a good sense of security? Or does it mean me and my SO are just inevitably doomed, destined to break up? Am I a bad person?