Don't worry about it! Every child is different and you will know when your daughter is ready to stop wearing a diaper at night when she consistently wakes up dry or simply starts waking up to use the bathroom. In the meantime, just have her wear a diaper (or pull-up if she prefers, although they don't hold as much) at nap and night time and on long car rides if you think she is likely to fall asleep. Also, I recommend getting one of those vinyl mattress covers that zips all around the mattress to protect it. Top it off with a waterproof but cotton-topped mattress cover (that goes on like a fitted sheet -- get two if you can so you have one at the ready in case of accidents; Kohl's, if there's one near you, carries them and they often go on sale for about $20 a piece). This will keep the bed comfortable for her but protect the mattress for you.
I have three daughters 5 1/2, 4 and 13 months. My older girls have been out of diapers for 2 1/2 and 1 year respectively but both still need to wear a diaper at night. This is not uncommon. They both sleep hard and simply do not wake up when they need to go. I have gotten very negative feedback from some folks about this, but I would rather that my kids do not have to experience the shame and humiliation I felt as a child when I occasionally wet the bed (which is also normal). Granted, I would not shame them, but I can't say I would be all that pleasant, no matter how much I tried to be, if I had to get up in the middle of the night every night to change bed sheets, not to mention the sheer volume of laundry that would result. My oldest daughter is eager to be done with the diapers, but accepts our explanation that her body must not be ready. For a while we even tried cutting out letting her have a cup of water near her bed during the night, but, ironically, that would fully wake her because she would wake up thirsty.
I know lots of folks whose kids potty trained and simply didn't wake up during the night to go and still woke up dry, even when the kids were very young. But I also have a good friend whose 7-year-old daughter and 9-year-old son wet their beds almost every night. The boy especially is embarrassed and feels bad about himself because he does (the parents do not shame him -- in fact both parents also were bed wetters as children). My point is, every child is different, so you need to do what works best for your family.
One thing that it might help you to know is that the average age for kids to potty train during the day is something like 36 or 38 months. Your daughter is ahead of the curve for the daytime stuff and that's great. Something that may or may not happen (it did with my older one but not with my younger one) is that in a few months she may regress a bit and have a time period where she has a bunch of accidents. As I said, it may or may not happen, but I know it's pretty common, so don't be worried or surprised if it does. For my daughter, I think the novelty of using the potty wore off for a bit AND she saw that her younger sister was getting some "extra" attention during diaper changes. Every so often, she would refuse to wear her underwear and would ask for a diaper instead. We'd ask her if she was sure, and if she still asked to wear the diaper, we'd let her. We didn't make a big deal out of it and it rarely lasted even a whole day. This may be something you encounter with a new baby arriving at your home.
In the end, my best advice is to keep focusing on and being positive about the daytime stuff and let the sleep/potty use thing unfold over time. It will happen eventually and you and your daughter will probably feel a lot less anxiety about it if you just let that piece go for now. If it continues past an age where you are comfortable with it, talk with your pediatrician about possible causes and solutions such as medication, bed-wetter alarms, etc. (although these are not things I think any doc would recommend for a child this young).
When I was having my second child (my first two are only 18 months apart) a lot of people really pressured me to get my older one potty trained because, they said, it would just be so much work to have two in diapers. In the end, it really wasn't that much extra work at all. And, even if you are worried about the expense of having two in diapers, remember, you will be using only two or three diapers a day tops (nap and night and maybe one for a long car ride) for the older one, so you will go through them more slowly and spend less on them.
Finally, one last thing you may find helpful: if your daughter happens to get any stomach bug (there's one going around our community right now, which is why I mention this) and gets diarrhea while she is still working on getting totally potty proficient, offer her the chance to wear a diaper if she wants to. It might be a little scary for her to use the potty if she has loose bowels -- it's different than regular BMs -- and she might be frustrated if she doesn't make it to the bathroom in time (and it'll be a big mess for you). I wouldn't force it, but, if she's not feeling well, it can be a relief for her to just let go back to the familiar.
Good luck -- with the potty training and the new baby!