I agree with what most of tribute says, except for the comment about not working things out. Depending on her trimester, she may have substantial discomfort due to having a baby inside her! While I don't do "deep tissue" on pregnant women, I do "work things out" to some degree, using decreased pressure and no pressure points. There are so many pressure points to avoid, that I just avoid any kind of pressure point work. I do lots of rubbing, circular movements that are soothing and can be firm, but not DEEP.
I do a lot of extra checking in, and I let her know at the beginning of the massage that I will be doing this.
I think it's normal to feel apprehensive about pregnancy massage if you've never massaged a pregnant woman. But the fact is, (having at least some training in pregnancy massage) that as long as you don't work deeply, keep checking in with her, and don't use pressure points, you should be fine. If you don't know how to do sidelying and/or draping for this, then you're lucky that she's a relative because it's easy to lose the drape and that wouldn't be okay with a paying client. Just let her know ahead of time and have her wear undies if she's concerned.
As tribute said, have lots of extra pillows and I would add, have a few towels, to use to drape the breasts, or other areas while you are working out the draping thing. Good luck and you will do fine!