Harry was 21 months old when I had Grace and he was nowhere near ready to toilet train (he is still resisting the idea now). My advice to you would be to not worry about it until after you have had the baby, if all goes to plan and you have another bub when Lily is 21 months or so. IMO 18 months is too early, but all toddlers are different.
I actually think having the two in nappies was easier in the beginning, because I could just change them both and be done with it. The last thing I wanted to be doing when Grace was a newborn was trying to get Harry to use the toilet. Now both of them are older it feels like the right time to start training him, if only he would agree!

Bon