First things first, if you have this fear about your sister – examine where that is coming from. Then decide whether time with your sister is worth the anxiety of a possible ambush. It may very well be, only you can decide that. If it is, than you can prepare for the eventuality of an ambush.
If your parents do show up, what will you say and do?
When I was dealing with this, and was afraid of ambushes, I practiced a speech I could simply ramble out at them. I practiced, so I can remember, and actually say the words without any emotion. So, when I did had to use the speech (which I had to) they could not accuse me of being angry or unreasonable. I was factual and detached instead.
I had also decided that I was just going to leave. Should I call it a “Speech & Dash”? I walked away, and called my partner to vent, rant and rage about the whole affair.
How will you deal with the anxiety before you go there?
I take it you are (or will be) feeling nervous about this prospect. So how will you self-care and keep your nerves in check? I often talk a lot to fellow-survivors, my partner and my family of friends when I anticipate difficulties. I also make sure I take enough time to meditate, rest and do some nice things, like taking more walks or have a bath.
And when it is over?
Make a plan for the post-confrontation too. How will you care for you emotional state, and take care of the physical ramifications of stress? Will you plan your favorite meal, have movie night with the kids? Plan something good to look forward too after the visit to your sister. That way, the visit and possible ambush are not the end all and be all of the day, and you have something to recharge the battery after all the upheaval.