And you've kept that up for 18 days, wow. You must be a very patient person!
For the older pair of twins, put mum outside and let her dry off. Keep the lambs inside and bottle feed them 100%. They will soon start drinking more once they realise that you are now the provider of the milk.
For the other pair, you could do the same thing really. Or, leave them in with mum as she loves them and keep bottle feeding them. You might find they are getting some from mum when you aren't looking.