well piglet, i was thinking of you in the shower this morning. it's where i do my best thinking these days

and i came up with a creative little story for your dd as to why baby bro can nurse and she can't:
when you pour the last cup of milk (or juice or water) from the container, make a point of showing emily "look, we drank all of the juice. now there's no more in the container, we did such a good job of drinking it all. YUM!" then later after she has nursed, you can say that you had a talk with the boobies and they said they are almost empty of emily's milk, just like the empty juice container this morning. shall we have a big party to celebrate you drinking all of your wonderful mama milky? we could bake a cake and drink lots of water or milk or juice in a cup, just like mama and papa do! won' that be fun?
"and why, mama, is there still milk in the boobies for baby brother?"
"well, because emily did such a good job of drinking all of her milky that she drank it all up, baby brother is still working on drinking up HIS milky. and one day he will drink his all up too, just like you did"
it's not entirely honest, but i like it because it CELEBRATES her moving on to a new phase of her life, and it also allows baby brother to drink "his" milk without making any specific judgements about babies nursing and big kids not nursing. plus, i think it would make sense to a child her age. if you want to wean her gradually, you might suggest she only nurse at nap time & bedtime (or whenever) so she can make her milky last longer, then you can every so often talk about how emily's milky is almost gone, soon it will be all gone, etc.
just a thought...
also for those who's children seem to grab and scream when you try to cut out certain nursing sessions, i think it has a lot to do with COMPLETE consistency. that is if you are committed to only nursing at bedtime and your child falls down and gets hurt, and you choose to nurse him/her then, you are actually confusing them by nursing then, IMO. it's a perfect opportunity to practice new parenting skills (those that don't involve nursing as The method of comfort), and if you are always consistent, your child will learn that nursing is something that is only done at certain times of the day. THIS IS NOT A TERRIBLE THING TO DO!!!! i don't know why we feel so guilty about it. i don't think for a second that any mama here would not be offerring comfort to a child who is hurt. there are so many other ways to offer comfort other than nursing! nursing may be the currently preferred way, and it may be the way you are used to, but if you gradually intorduce new ways, these will be just as accepted to yoru child. i know, i've BTDT with my 1st.