The scene that has lingered in my memory for a few days...(spoiler)

I can see why one might think she used the baby in a spell to make herself appear young. If one isn't familiar with witch lore, it may not be apparent that the baby is being used to make "flying ointment" or "Witches Unguent".

We see the witch rise into the air against the backdrop of the full moon after she rubs the ointment all over her body and the broomstick. But without knowing baby fat was believed to be part of the recipe for flying ointment (according to primary sources of the early modern period), it isn't apparent as far as the plot is concerned as to why the witch would take the baby and use it's rendered fat to smear all over her body if not to further the story involving the family. I.e. Make herself appear young so she can bewitch Caleb thus preying upon the family more.

But that's not the case. The witch took the baby to make "flying ointment". The director spent several years studying the lore and primary sources of the period. And according to some of the sources, that's how "flying ointment" was made and what it was used for (or so they believed). So it's inclusion in the film gives a horrifying introduction to the evil it was believed witches did as well as creating the first horror committed against the family by the witch.

If you didn't know how witches allegedly were able to fly on a broomstick, you do know and it's truly horrifying.

I loved this film. Brilliant.

Thank you for making witches scary again.

P.S. Wiccans, I know witches aren't really.

She is seen rubbing the guts on her broomstick and body. She only appeared to look that way to Caleb in order to lure him to the dark side. Heh heh.

The witch that kills the baby is old, the witch that Caleb finds is young. Both seem to have the same cloak. The witch used the baby to make herself youthful again.

I'm pretty sure it's flying ointment. I won't say definitively, but I'd bet my bank accounts on it. She uses it on her stick also, which she then uses to fly to the moon or a Sabbat that was taking place.

In the scene with Caleb, she is not really a young woman. It's a charm or a glamour, and Caleb falls for it (since he's already having trouble keeping his eyes off his budding sister) but at the end of that shot, the arm that draws Caleb closer is that of an old crone's.

In the real world version of flying ointment, historians believe that 'witches' used a number of psychotropic drugs which made them hallucinate that they could fly on their brooms or what have you.

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"It's better not to know so much about what things mean." David Lynch

The witch that kills the baby is old, the witch that Caleb finds is young. Both seem to have the same cloak. The witch used the baby to make herself youthful again.

There's clearly more than one witch. We see a whole group of them at the end. And we see an image of the witch flying in the air after the baby is killed.