There appears to be some information missing in your post. You state that you brother was found guilty of a Section 18 offence and a section 20 offence as well but you've not told us how (if at all) the Section 20 offence was related to the more serious Section 18 one.
The 'starting point' sentence for a Section 18 offence, committed by a first-time offender convicted after a trial, where a weapon was used which came to hand at the scene and where the victim did not suffer very serious or life-threatening injuries is 5 years imprisonment. Judges are obliged (unless there are genuinely exceptional circumstances) to pass a sentence in the range of 4 to 6 years. The element of provocation might tend to push the sentence downwards but any previous convictions (and/or the Section 20 offence) might push it higher.
Based on that, a sentence of around
5 years seems to be quite likely.
Source:
http://www.sentencing-guidelines.gov.uk/docs/a ssault-against-the%20person.pdf
(Refer to the table on Page 13).
Chris