Problems with weaning from bottle

[babyonboard]

My DS is 17 months and was doing well with cups until his sister was born (she is now 5m). We had cut him back to just a nightime bottle of milk and during the day he had sippy cups or assisted drinking from a 'proper' cup/glass. Now he wants DD's bottle whenever he sees me feeding her, and refuses to drink from his cup.
I've had some success with giving him straws to drink with, but he still seems fixated on her bottles.
I understand this may be a more complex jealousy problem, but am not sure how to deal with it.
We have tried explaining to him that bottles are for babies, but he is too young to fully understand.
She has a dummy, which he also had until 3 months old and he is not remotely interested in that, so I just can't see why the bottles are causing us so much fuss.
Should I try weaning DD onto a cup, she seems too young but my HV has suggested this!?
If anyone has experienced something similar, or may have some input otherwise I'd be really grateful