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My sister was TTC when we went shopping for bridesmaid dresses for my wedding. She already felt more than comfortable telling me the plans, so when we went shopping, we looked for something that would work both ways (look good preg or not) or something that had two options-- one that looked good now, and a maternity option. Then she ordered as late as possible. She ended up being 5-6 months pregnant at the wedding and wore 2 sizes larger in an empire waist dress, but she also said it wouldn't have zipped up a week later.
If you are comfortable, I'd explain the sitation to her and ask the latest you could order. Usually it's only about 4 months for a BM dress, so if you weren't already pregnant at that point, you should be fine in whatever size you order. It's harder if you don't want to tell her about TTC though.

Just remember that the dress stores deal with pregnant (or possibly pregnant) bridesmaids ALL THE TIME. Weddings and babies happen at the same time for a lot of families/close friends so they are perfectly used to it. If you don't want to share your TTC with your SIL, maybe you can still talk to the dress store privately about your options for the BM dress, and see what they suggest.

I was in my SILs wedding 11 weeks after having my LO. I bought my dress in a size up so worst case all I would have to do is have it taken in. Turns out I didn't even need to do that.

I agree with @highwire: the dress shop we went to helped me decide what I needed to do and what my options were as far as alterations etc went with the specific dress we had.

My best friend (we've been close since we were 5) and her husband (who gave me away at my wedding) decided to TCC three months after I'd asked them to stand in my wedding (and a year before the wedding!). I'm going to confess something here - I was really pissed for, like, a month (until she got pregnant - and then lost the baby - then I realised it wasn't all about me and stopped being so selfish!). I didn't tell her I was upset, but boy was I miffed..... (wow, what a horrible person i actually am!!)

So, advice number 1: I think you need to let your sister know sooner rather than later that you're TTC. I'm sure she'll be really happy for you!

Anyway, I picked long black gowns made out of clingy, stretchy, lycra fabric. I bought hers several sizes bigger (she wasn't actually pregnant when we bought the dresses) and had a seamstress take it in a couple of weeks before (she was only 6 weeks pregnant at my wedding!).

Advice number 2: It's much easier to take dresses in than make them bigger so make sure, whatever the design, yours is ordered in a larger size !