You've said several times that "we feel that we dont want to let them ruin things for us (again)". That statement implies that you and DH are a united front and on the same page with regard to dealing with his FOO. However, from what you've described, that isn't entirely true.
You've said several times that DH feels very guilty about reducing contact with MIL and SIL, that he hopes to resume contact with them, that you're worried about SIL reeling him back in and H reverting to how things used to be, and that H basically doesn't know what to do about the situation.
To be honest, it sounds like the main problem for you on this trip potentially won't be SIL (should she decide to go), but DH. His behavior with regard to SIL and his FOO is an unknown. I think you're wise to start thinking about how you'll handle it if you're on this trip halfway around the world from home and DH gets reeled back into his FOO's shenanigans.
How is DH’s supposed to react towards SIL if I am in his company?
He's supposed to put you, his wife, first. His relationship with you is his priority, not his relationship with his sister. That's not to say he should ignore her, but when interacting with her, he should always come from that standpoint. His first loyalty is to you, his wife, and not to SIL or the rest of his FOO. His attitude, from day one forward, should be "This is my wife, and I will not let her be disrespected by anyone." Not that he necessarily speaks those words, but he conveys the message by his actions and demeanor.
I feel really sorry for DH, he is going to be in a real bad predicament if she does this,
He will only be in a "bad predicament" if he's still in the FOG.
if he joins in with the show will I be wrong to be angry with him?
By "joins in with the show", what exactly are you referring to? Does it involves DH getting caught up in SIL's drama, especially if it involves her mistreating you (even covertly or plausibly deniable) and him not standing up for you?
Can you talk with DH now about your concerns, in advance of the trip? You and H together can make a plan to handle it in case things start to go awry with SIL. That way, you wouldn't have to wait until H got reeled in and you got angry. Plan now so you and H can work together so it doesn't reach that point.