...qui se tam grave...

Orberg LLPSI Cap XLIII

Hac lege duumviri creati sunt, iudices severissimi, qui se tam grave crimen neglegere posse non rebantur, et Horatium condemnaverunt. Tum alter ex iis “Publi Horati, tibi perduellionem iudico” inquit. “I, lictor, colliga manus!” Accesserat lictor iniciebatque laqueum collo.

I’m just a little unsure of ‘…se tam grave..’ Is this making rebantur reflexive on the duumviri - as in they thought themselves unable to to ignore…?

Hence I would translate this as: ‘By this law the duumviri were created, the most severe judges, who thought themselves unable to ignore such a serious crime.’

I there’s anyone reason for ‘se’ then I’d be keen to hear it.

Your translation works, but se is simply the accusative subject of infinitive posse. This is indirect speech.

qui se tam grave crimen neglegere posse non rebantur – “who did not think that they could neglect such a serious crime.”

One of the syntactic differences between Latin and Greek.

Many thanks.