Hi - good question and I agree with you, OP, that it does feel annoying to get all the "sleep now" comments when you're pg with your first (especially), though I do honestly believe people have good intentions.
I've always been a crap sleeper anyway, so in a way it was a bit easier to adjust than it must be for those who love sleep/are champion sleepers. But nonetheless there were days when I felt dizzy with sleep deprivation which is unusual for me. Like lots of others here have said - it's cumulative. After 4mths of co-sleeping to BF I felt a bit mad and took action, eventually getting DS into his own room by about 6mths and getting him to settle himself into good sleep patterns using CC at about 8mths. He was one of those BFing babies that latched on every hour but I don't think he was feeding much. I hope you get a baby who is more like every 2 hours moving to 3-hour feeds in month 3 (if I remember it all rightly). I also adapted and began to fall asleep more quickly than I had in the past but this was often with a baby on my chest and I'm not convinced this was always a great thing to do for him or me...
If you are BFing there's no harm in introducing a bottle of either expressed BM or formula once the BFing is established and going well. (Or trying to - some babies won't have it...) This way your partner can take over one night feed which potentially means you can get a chunk of 4hrs sleep - better quality than the 1.5hrs you manage otherwise.
One trick I learned was to make sure that BFing during the day was a time to rest and not do anything else at all, whatsoever. It was also lovely bonding time with my DS. I'm expecting DC2 in 8wks and worry that with a toddler I won't be able to do this again, sadly.
Agree with the advice that you shouldn't feel obliged to do anything unless you want to and have the energy. Of course, my DS will make me obliged to play trains when I'm on my last legs, I'm sure, but no doubt I'll survive to tell the tale. As already said, it feels like forever but actually the first year of any child's life does seem to fly by because of all the wonderful changes you get to witness and learn about with matchsticks propping open your eyes.
Good luck. And go and have a rest now, you need all the sleep you can get before baby arrives! 
ps wishing TwinkleReturns all the best with solo-parenting - hope you have some family/friends to support you and that your toddler is exemplary in behaviour with sibling 