Sounds like a security and control issue to me. I am a 56 year old, heartless wonder, grandmother.
A one year old with a bottle is fine, at two it is time to weed it away. Sounds like you are trying to rush the younger one.
Heartless as it sounds if there are no bottles in the house then they will have to give them up, won't they? If you want them to stop drinking milk out of a bottle then simply do not put milk in a bottle. The only time I would give a bottle to anyone would be at nap or bedtime and then it would be water anyway, not milk that would spoil especially if warmed up.
Believe me, if you refuse to allow yourself to be controlled now you are saving a lot of problems in the future. Children will drink their milk if it is the only beverage offered and if they are thirsty. Sippy cup, straw cup, water bottle with the pull up top, whatever dispenser it is in; and make that the only available drink. As for heating it up, if you are tired of doing so then lessen the amount of time it is heated by 5 seconds a day for the next week. Again, they will drink it if they are thirsty and it is the only available beverage.
My grandson didn't like milk after being breast fed so I learned to put a little flavoring in it, a little vanilla, chocolate, or strawberry flavoring, not syrup and he did well. Personally I prefer whole milk, which when I buy it at the store looks and tastes entirely different than the dairy milk I was raised on where the whole milk had cream that settled at the top and had to be shaken back into it when I and my children were growing up. I understand you can get the vitamins and minerals in milk from other sources but I think the advantages of getting these from the real product rather than a pill is the best way.
P. R