Hey, In Christ!
Congrats again on the babies! I have a 7 yr old girl, a 2 yr old girl, and a 9 month old boy. I've been down this road a time or three. ;)
You've been given great advice, especially from AirDancer. Every pregnancy is different. My wife didn't have these issues with our first and third, but your wife's situation sounds a lot like our second. Your wife is wracked with hormones and emotions that she can't control. Chances are even she can't really understand them. First and foremost, don't take it personally. I know some people spout about women's hormones being an excuse, but that is one thousand percent true in this case. It's really not her fault.
Second, like AirDancer said, the best thing you can do for her right now is make her feel special. Let her know you love her, and that you're there. I don't know how physical you usually are (I mean hugs and kisses and the like), but about a half dozen extra non-sexual hugs every day will go farther than you think. It's just a small thing that sends a big message to her. "I'm here, and I love you."
I know all women are different, but when it comes to "love and romance", my wife is very much what you'd call a "girly girl". By that I mean the smallest things I do send the biggest messages. Extra hugs, lean over and kiss her forehead for no reason at all. Little notes she'll find during the day. Stop by on the way home from work and get a pack of Reese's cups (her favorite candy). She's a big Nightmare Before Christmas fan, and once in the store I saw a book mark that had one of the characters on it. I bought it and she nearly cried when I gave it to her. My point is you know your wife's "Love Languages". Do everything you can to tell her you love her in every one of them.
Your wife's mind and body are pulling her 1000 different ways right now. Even if she's not showing appreciation, trust me, if you show her you're the one constant in your house right now, you will go a long way to helping her emotional and mental state.
I know I'm getting long-winded, but I'd say one of the most important things to do until the babies arrive is make time for just the two of you. Have a date night or three before "crunch time". It doesn't have to be fancy, it just needs to be the two of you, with you courting her just like you did before marriage.
I'll finish with this - intimacy. I know as a man it's rough on our transmission when we don't drive the car for awhile. If it's a crucial thing for you - which it may well be - just talk to her lovingly. If you then get intimate, do everything you can to help her relax and get comfortable before hand. Candles, a massage, well - I doubt you need instruction there, you know what I'm saying. But if you're not, again - don't take it personally. Once the babies arrive and her body begins to get back to normal, that will come again. It's definitely not like it'll never happen again.
And - mostly because I think you need a chuckle, but also because I tell all expecting dad's this...
Assuming the birth is natural (not a c-section), your wife will probably tear and need a few stitches. If that's the case, don't believe the doctors when they tell you to wait 6 weeks before getting intimate again. Wait until she has a checkup afterward and you can be 100% certain the stitches are gone. I won't be crude or nasty, but they don't always dissolve in 6 weeks.
After our first daughter, we waited 9 weeks, "just to be on the safe side". Those "gone in 6 weeks" stitches were still there, and I have a scar to prove it.