Her Body Broke the Best

[Leanne]

I see you've met my sister... 

You have some excellent lines here, especially in the first and last stanzas.  I do feel you're let down by your very stilted, uneven meter and some awkward phrasing.  Those are not difficult things to fix but they do take a while to learn and I'm not going to rewrite this for you.  I am going to suggest you take some time to find out more about how meter and rhyme work together, or even meter in blank verse and get rid of the rhymes.

She set upon her gut a cauldron, -- great opening line
fashioned it with straps. -- this sounds as though she's made it out of straps, although I presume you mean that she's put straps on it.  And if my presumption is wrong (or if it isn't, because it easily could have been), consider the dangers of ambiguity in grammar...
In it poured her lover's tallow, -- you could consider "poured into it her lover's tallow", for meter and to remove another unnecessary inversion
a slush of rot and fat. 

With mythic wave and incantation, -- why is the wave mythic?  Didn't it happen?  Maybe mystic is better.
a spell she did in-set. -- this syntax is just dreadful
Within a span of nine moons past, -- using "past" makes it sound as though she's made herself pregnant 9 months before she did the spell, and I don't think that's what you mean.  I'm assuming that from the time of the spell, nine months have passed.  You could try something like "and when nine moons had crowned and set".
lost love the witch begat. 

Necromancy the town's folk cried. 
Blood magic bellowed the priest. -- these two lines are quite bad for meter.  I don't see "necromancy" fitting in there at all, I'm afraid.  To be honest, it's probably too fancy a word for small-minded folk anyway Maybe something about the devil or a demon.
Hang the witch dear sister beckoned,
dismember her at least. 

Disfigured form and blood-lust haze,
lost lover would protect her.
Flesh Golem bound to woman heart, -- woman's?  Also, "flesh" is throwing the meter off -- is it needed?
a fury forced encounter. 

The town's folk lay in bloody pattern, -- I'd like to see you use a one-syllable word rather than "pattern" if possible, and maybe try to phrase it so you don't have to keep leaving out articles, because that sounds as though you're not in control of your poem.
the priest a sodded mess.
Dear sister strewn upon the floor,
her body broke the best. -- a powerful closing line, and quite a satisfyingly gory conclusion