I am sorry to hear that you are in such a situation - I think that someone getting involved with a "newly poly" couple, no matter how they came into it, is very hard on that person, because the established couple need to work out what they want, and that can often change and be very emotional.
Lying is tough to deal with at the best of times, and in situations like this it's even harder. Any relationship, but especially poly ones, are about mutual trust - a sense of security can only come when everyone involved can trust each other. So from what you are saying I think there is a lot of building to do in that regard.
if you are uncomfortable with the lying (and I know I would be) then make sure that stops, in the sense that nothing happens that you have ot lie about, or be afraid you will get "caught" - put the lies in the past and don't make them worse.
And if I could recommend some further reading to you - there is a guy who writes many wonderful advice articles on poly called Franklin, and he has some good things to say about "secondaries", which it sounds like you are heading for. Check out his article at http://www.xeromag.com/fvsecondary.html
where he talks about some of the things that you need to think about.
The rest of that site has some great reading as well - you might want to recommend it to all involved.
I really wish you luck on what seems to be a challenging path going forward.