Hi Robbin, I had a similar (not exactly the same) situation last year at Christmas (ahhh, the holidays...). My dog is 95lbs, dominant and possessive/territorial (not in an extreme way, but it's there for sure). My family had a 17 year old Pomeranian who Oscar had grown up spending some time with, but pretty much we just taught him to ignore the little guy (who was VERY meek, and obviously geriatric - he wasn't picking any fights, that's for sure!). As soon as Oscar hit adolescence he decided that my parents house (the Pom's house) was actually HIS house and that he needed to assert his authority over the little dog. We didn't quite see this coming (and since Oscar was in the middle of serious mental growth stages - it seemed to happen very quickly) but Oscar just went for the Pom one day and he could have easily been toast. Thankfully, his copious fur kept him from puncture wounds, but from that day on we just separated them whenever we came over for dinner or whatnot - NO interaction, not even glancing, there was a door between them at all times.
Well, last Christmas I went home for a WEEK, with Oscar. I was right in your shoes - how on earth was I supposed to create a comfortable environment for 2 dogs that couldn't be near each other, 24 hours a day, for 5 days?? I asked the board. Their collective wisdom: bring a crate, crate Oscar when in the house and just make sure he got lots of exercise OUTside every chance I could. And that's just what I did.
It wasn't what I wanted (obviously, in a perfect world Oscar would love Eddie (the Pom) and snuggle with him at the bottom of the Christmas tree, and they could share gingerbread cookies while opening their presents...) but the perfect world was already out of the question, so the next best thing - in interest of keeping everyone SAFE and the humans SANE - was to crate the big boy and let the little old man have freedom in his own house (Oscar really had no right to be free in the house if he couldn't respect it's inhabitants, because it WASN'T his house...). My mom was thoroughly disgruntled at having a gorilla cage in her living room, but if Oscar couldn't be loose, he would at least be in the same room as the people. Eddie stayed at the other end of the room on his own but seemed to know he was safe.
All in all Robbin, there's no cheery solution here, the circumstances just don't permit it. Fine. We're grown ups, we can handle that. What I would suggest (in my humble opinion) is that you think about this FIRST from the standpoint of the SAFETY of ALL the dogs (no matter how obnoxious they are, the terriers DO deserve not to be terrorized in their own home - and Levi doesn't deserve to be pestered by THEM). Your own convenience should come second to this. While it is easy to imagine tethering Levi to you, he may actually feel more relaxed if he is in his crate - a safe place (assuming he has one and likes it already... and assuming the terriers aren't allowed to bait him from outside it...). Plus, there is no room for error on your part if Levi decides to lunge, no catching you off guard and pulling you off your chair in the middle of Thanksgiving dinner... in a room full of festive people, it's very hard to maintain the same kind of composure you would in a training scenario when your dog acts out. Levi's also at a place in his development now where you still don't even know what he's capable of, or how he's likely to react in all situations... please don't underestimate him...
The holidays are stressful for EVERYONE, including pets. Hubbub and commotion - and LOTS of distraction - can make it tought to concentrate and focus on even the best laid plans. Since the worst case scenario in this case - with one BIG dog-aggessive dog and 2 LITTLE baiting dogs - is EASILY physical harm or even death for the terriers (you said it yourself), I just wouldn't risk even a moment of opportunity - listen to that common sense!! If you only have to be there for an evening, maybe room separation could work - with vigilence - but if the stay is days long, I really suggest bringing Levi's crate... we have a crate that even collapses (huge as it is) into a neat, flat item that lies on the floor of the trunk of our car. Nevermind whatever poor behavior is exhibited by your in-laws dogs, nevermind that there are no rules for them - it isn't your house, or Levi's, you two just have to make it safely through a short stay there. Crating will let Levi have his own space, under your control, and you can make every trip outside an opportunity for continued training and structure.
Just my two cents.
Good luck with your trip!
~Natalya