NL25 6max - 2 pair on river

[ADK]

PokerRoom No-Limit Hold'em, $0.25 BB (5 handed)

SB ($13.75)
Hero ($25.50)
UTG ($29.55)
MP ($24.10)
Button ($26.80)

Preflop: Hero is BB with Q/images/graemlins/diamond.gif, 6/images/graemlins/heart.gif.
UTG calls $0.25, <font color="#666666">2 folds</font>, SB completes, Hero checks.

Flop: ($1) Q/images/graemlins/club.gif, 8/images/graemlins/heart.gif, 4/images/graemlins/diamond.gif <font color="#0000FF">(3 players)</font>
SB checks, <font color="#CC3333">Hero bets $1</font>, UTG calls $1, SB calls $1.

Turn: ($4) T/images/graemlins/club.gif <font color="#0000FF">(3 players)</font>
SB checks, Hero checks, UTG checks.

River: ($4) 6/images/graemlins/spade.gif <font color="#0000FF">(3 players)</font>
<font color="#CC3333">SB bets $12.5 (All-In)</font>, Hero ?

[Isura]

Fold. Looks like a straight or set that missed the turn c/r and is trying to make up value while looking like a bluff. Especially since its 3-handed the bluffing frequency is lower.