You won't harm your unborn baby by nursing. The fetus will take everything it needs from you. Your body will give whether or not you are strong enough to give, because that's how it works. Women do nurse while they are pregnant, but if you choose to do this, you really need to be healthy, and work with your doctor to ensure you stay healthy, and you need to get some rest.
Really and truly, you are only comforting your daughter by nursing her 4 times a night for a few minutes at a time. She doesn't need the milk. You don't mention how much she's nursing during the day - what about her solid food? Do some research into nursing at this age - you can read about it through La Leche or talk to a nursing specialist. DON'T let anyone make you feel like you are selfish or not a good mother for wanting to wean - it's your body, not theirs.
You will have nine months to make changes before the baby comes unless you want two children in the bed with you. The first thing you have to do is decide whether night time nursing is over. If it is, wear your bra to bed to give you resolve. Just keep turning away from her and pretend to be asleep. This will be hard, but you gotta do it. At some point she will stop waking up for it. After this is established, then put her into her own bed. I know you hate the cry it out approach, but there are different ways to do it. Read about the Ferber method. Instead of letting her cry without going in to her, you could go in and pat her on the back and go out (don't pick her up), then come in 5 minutes later, do the same (only stay for a few seconds, don't engage), come back in 10 minutes later, etc. Extend 5 minutes every time. You must actually watch the clock. It may take two weeks, but if you cave in and take her back to bed with you or pick her up, you might as well not even bother.
With the new baby, if you decide not to co-sleep, keep her in a bassinet in your room - not in your bed. Have a pad under her that you pick up with her to put her in your bed to nurse. Put her back in the bassinet after nursing her, with that warm pad still under her. It'll keep her from being cold when you lay her back in the bassinet and make it easier to lay her down away from you. After she weighs 12 pounds and the doctor clears it, move her into her own room in a regular size crib. Make sure she has enough nursing during the day - consider giving only one breast at a time so that she empties out an entire breast to get that thick hind milk. She'll sleep longer on that than the lighter front milk. If you want to get up and feed her in the middle of the night, try to keep it to once so that you aren't getting up all night long. At 12 pounds, she doesn't need to nurse all night long. At some point you can knock out the night feeding and then everyone will get much needed sleep.
I hope you will at the very least stop the nightime nursing. At least you'll have a shot at getting some sleep. You really need rest to give your new pregnancy the same TLC you gave your first pregnancy. Try to remember that when you are teaching your 17 month old to self-soothe and do without you in the middle of the night.
Good luck and all my best,