Alright Marie - I had a brainwave of an idea!
I used to water down DD's bottles when weaning feeds that she was reluctant to give up so why not do it in reverse??
Both of these girls are fine with water in the sippy... maybe try a dilute version of their milk, like 4 ozs water, 1 oz milk/formula?? Then, over time, reduce the water and increase the milk/formula???
I am gonna give it a shot in the morning and will let you know if DD shoots me down!