If you have limited side bend, then you will also be limited in your spinal rotation because one motion cannot happen without the other motion. Any limitations in this area will cause you to slide or sway in your golf swing. Improving your range of motion you can side bend will help you make a better backswing and improve your swing plane to be more consistent.

Stand with feet together. Keeping your back, head and shoulder in the same line, slide your hand down the side of your leg.
Go as far as you can without losing your balance.
Hold position for 8 to 10 counts and return to start position.
Repeat on opposite side.
Do 8 to 10 repetitions on both sides.
You should be able to reach your knee joint on both sides.