Re: DD3 is 15 months and nurses often through the night
Eventually your baby will cut back on the night-nursing. She's not going to be 7 years old and still nursing through the night, right?! The issue with self-weaning from night nursing is that it doesn't always happen in a timeframe that works for mom. If night-nursing is making you feel burned out, then you might want to consider taking a mother-led approach to night-weaning. It might help to start by offering to nurse more during the day- if your LO is making up for missed daytime nursing by staying latching on all night long, more daytime nursing could reduce her nighttime need.
There's no way to tell how your particular toddler would behave if a new sibling were in the picture. She might un-wean, she might not. Most kids don't un-wean unless it's only been a very short time since they last nursed. But you never know! Many, maybe even most (?) toddlers wean during pregnancy, often because the milk dries up and their interest in nursing does, too.
Nighttime nursing with a toddler and a newborn seems to be one of the more difficult challenges for a tandem-nursing mom. If you know that night-nursing a newborn is a big challenge for you, that might be another reason to encourage your toddler to wean sooner rather than later. But unless you feel driven to wean now, I don't see any reason why you should force the issue because of a baby that might be many months or maybe even years in the future.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"