If the brothers are being referred to by 'they' (and I agree that they are), why does the grammar suggest otherwise? In terms of syntax, the Midianites are the antecedents of 'they'.
Another apparent oddity: Judges 8:24 feels the need to explain that the Midians were Ishmaelites (they wore gold earrings "because they were Ishmaelites"). As the words were purely 'synonymous' and 'interchangeable', why would such an explanation be necessary - especially since the author has already used the terms interchangeably without feeling the need for such an explanation back in Genesis 37?
If we take the Bible to be the word of God, this would suggest that a ) God's syntax was careless; and b ) she had forgotten that she'd used Midianites and Ishmaelites synonymously, and without explanation, in an earlier book by the time she came to Judges 8.
An alternative explanation is this. The Old Testament comprises a number of traditional narratives that have been interwoven by editors. German scholarship in the 19th century unpicked the different narrative threads and was able to explain many of the apparent contradictions in the Bible (somebody blogs about it here if anyone's interested). The confused syntax and terminology in Genesis 37 stems from the stitching together of two texts, the 'E text' (from 'Elohist', due to this narrative employing the generic name 'Elohim' for 'God') and the 'J text' (from 'Yahwist', as this narrative uses the term 'Yahweh' - Jehovah - the Israelites' name for God, translated in many English Bibles as 'Lord God' whereas 'Elohim' is translated as simply 'God').
It is due to different versions of fundamentally the same stories being knit together that we have 'God' creating humans, male and female, in Genesis 1:27, and 'Lord God' creating woman from Adam's rib in Genesis 2:22; and a number of the confusing elements elsewhere, including the Flood narrative.