It was initially called The Brothers which would seem to suggest Sidney would have a big part in it though he only appears very briefly. He's described positively when he does appear and he's polite with a non flashy carriage so that's probably as good an indication as any 
Mr. Sidney Parker, driving his servant in a very neat carriage, was soon opposite to them, and they all stopped for a few minutes. The manners of the Parkers were always pleasant among themselves; and it was a very friendly meeting between Sidney and his sister-in-law, who was most kindly taking it for granted that he was on his way to Trafalgar House. This he declined, however. He was "just come from Eastbourne proposing to spend two or three days, as it might happen, at Sanditon but the hotel must be his quarters. He was expecting to be joined there by a friend or two.
The rest was common enquiries and remarks, with kind notice of little Mary, and a very well-bred bow and proper address to Miss Heywood on her being named to him.
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