H.P.
Hi N.. Don't feel bad. 18 mo. olds can't quite verbalize what they are feeling. Kids go through so much change in those first couple of years, but they are resilient & learn quickly.
I recently had a baby as well. I also have a 10 yr old, 3 & 2 yr olds. My 3 & 2 yr olds are 16 mo. apart. When I was pregnant with my now 2 yr old (Jude) I showed my now 3 yr old (Aydan) that a baby will be coming. I showed him with one of my daughter's old baby dolls how to be loving, nurturing & caring to the new baby. I showed him how I would nurse the baby, change his diapers, cradle & rock him, etc. I told him that I did the same for him when he was a little baby. He caught on quite well & had no jealousy issues. I did the same with Jude when I was pregnant with my now 3.5 mo. old (Ryker). Jude is very loving to Ryker,so, I think it helped a great deal.(Now, my mom doesn't like my boy playing with a baby doll, but I knew a therapist who told me that there was nothing wrong with a boy playing with them because it teaches boys to have that nurturing quality). Sometimes I do catch Jude trying to poke Baby Ryker, but I correct him by saying " No, Jude, baby doesn't like owies". Then I cradle the baby & say, "I love you baby" I will then do the same with Jude by giving him a big hug & telling him I love him. Jude turns around, hugs & kisses the baby & tells him "AW, baby so cute. I love you baby".
It's not too late. I think showing her with a baby doll may help. Little ones learn a lot by play.
Don't worry & good luck!