E was 17 months old when we brought D home from the hospital, and D was 28 months when we brought N home, so I've been where you are...twice! ;) The best advice I received was to defer to your older child for awhile. What I mean by that is that your baby won't know if you are choosing someone over him/her....but your big kid will. Just for awhile, if your big kid needs something, wants to show you something, is sad, whatever, give him your attention first and foremost as best you can. Obviously, if the baby needs feeding or is screaming it's head off, then this advice is useless. But during the down times, let your older child know you still see him, notice him, want him by giving him your attention. The baby can sleep and not notice anything at all...but your first born will be made to feel so much more secure if he sees that his Mommy still knows he is there.
Another tip I'd pass along is to just give yourself and your whole little family some grace in those first sleepy weeks. Your big kid will act out in one way or another. Might be misbehaving, might be lots of whining, might be fits, might be a little bit of everything. It's just going to happen. The degree to which it happens is a variable you won't be able to predict, but, if you go into it knowing your kiddo is bound to have some struggles, then you won't be so flabbergasted when it does happen. Extend him some grace. His whole world is different now, too.
Extend your hubby some grace. Chances are, he's going to feel like the last man on the totem pole for awhile while people rightly spend time fussing over you and the new baby and the older brother. Remember him. Let him help in any way he can or wants to, and, biggest tip here, let him take the lead on parenting your big kiddo for awhile. You need the time with your baby to bond and rest and snuggle and learn each other. This is the PERFECT time to let Dad take over and have some special time with big brother! They'll both love it, I'm sure.
Most importantly, extend YOURSELF lots and lots...and lots and lots...of grace. All of this will be new to you, too. You need time to heal, let your hormones work themselves into order, and to learn your new baby. This was so hard for me at times, but let others help you in the way they want to help. So what if your clean dishes end up scattered all over your kitchen after a helping hand puts them away for you. Consider it a fun scavenger hunt. ;) If your family member is a good baby holder, run for a quick shower. If someone offers to clean, let them. Sweet heavens, let them! Everyone wants to help, and that help looks different on everyone. I thought I needed to prove to everyone that I could do all the things the first time I brought home a baby. Oh, silly, silly Erin. :)
Lastly, your littles will learn to coexist happily over time. The best gift you can give your child is a sibling, because they learn on each other how to behave in the world around them. The first time I caught a glimpse of my eldest girls starting to actually play together independently of me was like receiving the biggest pay check in the world. Your kiddos are so blessed to have one another.
Just push through those sleepy, blurry first few weeks and months, all the while extending everyone around you a bit of grace. None of you know what to do, and you are learning together...finding your new normal as a family of FOUR!