OK. So first of all, a h isn't dropped. It's added.
Piuthar is a feminine noun. That means any adjective following and directly linked to it get lenited (a h added). So "a big sister" would be "tha piuthar mhòr".
However, if you were to say something like "a sister is big", then the adjective "big" WOULDN'T be directly linked to the noun. Therefore, "a sister is big" would be "tha piuthar mòr".
Piuthar on it's own is "sister" or "a sister". If you were to say THE sister you would add a' and then a h to piuthar - so a' phiuthar.