Incorrect move.

(CodeCat moved page Reconstruction:Proto-Celtic/swekru to Reconstruction:Proto-Celtic/swekrū: I'm guessing this is the actual form, though I don't know if such an inflection existed in Proto-Celtic) (undo | thank)

Avoid guessing, maybe? This isn't some one-off thing, sometimes the Proto-Celtic form had a shorter u, as evidenced by the descendants. Swekrù leads to *chwygr, while Swekru leads to the correct chwegr.

How do you figure?

It's reinforced by what the addended Matasovic says, possibly even the original.

How can a word for a family member be neuter?

Good point.

It seems likely they actually imitated the word for father-in-law, which is why it's the form it is. So swekrū is correct.