hi hun, i wouldn't be concerned at all! a mate of mine has a little boy coming upto two years now, and he didn't talk at all until about 18-19months, when all of a sudden he learned lots of words all at once!
Another friend of mine has a 2 and a bit year old who says very very little, i was convinced he was autistic as he has no attention span, won't play with anybody, plays "deaf" never responds to his name or anything anybody says to him, no eye contact... pretty much every symptom of autism, but my mate saught advice from the HV who said "oh he's just a little bit behind, he'll catch up".... so... i wouldnt worry!
Sounds as though he's progressing fine for his age, ignore you mum and sister honey!