Ok, here's a couple quick tips:
If she tries grabbing your feet, it should become telegraphed eventually. From this point, you can either press




(this avoids lows, and sets up for a launching tech trap), or go into Base Hold Stance (



). It SHOULD auto guard impact. If not, you could feel adventurous, and try



from Base hold, which jumps over low attacks, and counters with a low of your own.
While Ivy is in whip state, she's most likely going to try some long range pokes. A good counter to this is to go into Chief Hold (

or




) which sidesteps, then covering the range between the two of you with Siegfried's Terror Slap (



from Chief Hold). It has great range, and punishes sidestepping nicely.
Otherwise, stay out of her close range! She has some hard hitting mids and stuns from that position, so play off of Siegfried's strength, which is his mid game. Use some of the same tactics as I listed above to deal with her from there, and then mix in some




~etc. combos, including some manual stance changes of your own choice. Specifically, use the


or



~

~

combo to rack up major damage on counter. Furthermore, avoid sidestepping around her when she's in whip state; her mids and lows hurt a lot off counter. Go for Sieg's poking attacks, like


,


, or


.