Hi, every situation is different. I have a 6 year old and now a 2 year old and they both usually go to bed at the same time(8 p.m.). The most important thing is trying to set a routine. Figure out a time that you can be consistent with and in order for it to work consisitency is key.
This is what I did when my I was nursing my little one. My son would get ready for bed brush his teeth and choose a book for me to read him. Sometimes I read to my daughter, theb I nurse my daughter in the room without interruption complete silence and the room dark. I put her to bed semi swaddled(with the swaddle me blanket and her arms are out, so she can flip back and forth. Check with ur doc if your going to do that but mine said it was fine.) so my daughter would go to sleep, i get one and one time with my son, we read the book and he goes to sleep.
At first when creating the routine that works for you they probably would not listen. Maybe when you first begin have your older one skip a nap and make them both tired during the day thay way they will go to sleep with ease. Also if they get up do not speak just take them back to their bed the first time just tell them its bed time then after that don't say anything just continue putting them back. Even ur little one, when waking up to nurse, it is hard aometimes but try not to talk or interact too much that way u could put the baby back to bed. In the first few days or a week it mau seem like its not working but stick with it consistency same routine before going to bed everyday and they would get it.
Also some tips that might help.
Avoid putting the baby down for a nap by 4/5 pm.
Before the designated bed time try winding down the day. Keeping the tv off a while before.
I know this is a lot but It helped me, and thankfully I was able to sleep good when both my kids were babies.
I hope this works for you. Also try asking, ur pediatrician may have some tips.
If you need some more tips i would be happy to help.