The SB cold called 1.5 SB preflop. So his most likely holdings are a pocketpair, high cards or a suited ace.
With so many players involved, it is very unlikely, that the SB makes this play without an ace. The flop check raise was intended to find out, who had the ace. If the bet came from the initial raiser, than he had to suggest, that MP1's kicker is stronger than his own one. But as the bet came from you, he could hope, that your kicker was not very strong, so he could continue betting out.

I would fold on the turn UI, but later I would always have doubts, whether I had made a correct play. [img]/images/graemlins/confused.gif[/img]