It's not clear from the question as to whether Mr's M's niece is pleading 'not guilty' and seeking a witness to support that plea or whether she's simply seeking a character
reference for sentencing purposes.
A court would normally only permit the use of a 'character witness' in a drink-driving trial if their evidence might show that the defendant would (for example) never touch alcohol (and that, therefore, their submission that their orange juice was spiked with vodka was probably true). e.g a devout Muslim might call upon his Imam to testify that he would rather die than drink alcohol. Other than in similar circumstances, I can think of no reason why 'character evidence' would be permitted within a drink-driving trial.
If a 'character reference' is required solely with respect to sentencing, then anyone (including family members) can submit such a reference but the court would almost certainly give less weight to a reference from a family member than to one from, say, an employer. Further, such references are usually submitted in writing, rather than by attendance at court:
http://www.ehow.co.uk/how_8162314_write-character-reference-court.html