D.H.
I'm an only child.....not for my not trying, but it just couldn't happen......I hated it.......I wanted a sibling so bad.......now my parents are gone and I have no one to look back with about my childhood except people who didn't live in the house.....
So, I say go for it......you obviously want one, and waiting till you can afford one will never happen most likely......18 months is a bit close.....but it would be 9 months......so..........
What does your hubby or mate think about this idea? That's a thought as well......
Good Luck and take care.........I'm so sorry about the loss of your siblings.....I can't imagine how hard that must have been......but you did have them to grow up with......remember the great memories.....