Sara Murphy was not cursed, as she was female
As speculated heavily by fans around the internet as to why Sara Murphy wasn't cursed. 1, 2, 3
Various conclusions have been reached, some being right guessing it's because she was female, some believed her to be adopted, a step sister, etc. Some went so far as to suggest that the clover in her hair was responsible for keeping her lucky. However, as confirmed by creator Dan Povenmire, the curse was in fact only hereditary along the male line
Screw Loose (@Scrw_Loose): Questions for @mmonogram and @DanPovenmire about the Murphy bloodline: Is the "disaster gene" passed on only to male of the Murphy lineage
Dan Povenmire (@DanPovenmire): Exactly
This is reconfirmed on tumblr, by story board artist Ashley Michelle Simpson
Someone on Twitter asked about the Murphy Curse and a few others mentioned in my inbox that it was strange only Milo and his Father have the curse and not Sara.
The curse only effects the male Murphy members in the family. I know they never make it super clear in the show. Certain things some characters say tend to hint at it, but yeah, it will confuse if questions aren’t asked.
So yeah, only Murphy men are born with the curse.
Ashley Micelle Simpson confirms the curse affects males only - KicsterAsh's Tumblr