Well, in English we usually use the present for the second part of the sentence:
- 'She will go if it's fine',
whereas Welsh uses the future. See the notes for this section.
Well... the meaning is the same, but the two instances refer to two separate things - 'she', a person, and 'it', the weather.
Just the context and the content. braf is usually used of the weather, and hi is always used when referring to the weather. The first hi doesn't then make much sense unless it refers to a person (or maybe to whatever other feminine entity which will go or not go somewhere depending in the weather).
If you were trying to convey some other, less commonplace meaning, you would probably use some other wording.