I was actually just coming here to make an almost-identical post! I don't have any advice, since I'm in a similar boat, but I wanted to let you know you're not alone!
DD1 will be 3 in April. She has always been all about nursing. As a baby, she couldn't even sleep unless she was comfort sucking the entire time. It's always been exhausting for me, but she obviously enjoys it so much that I've never been able to deny her. During my pregnancy with DD2 (who was born in Sept, when DD1 was 2.5 yrs old), she gradually decreased to 4 nursing sessions per day and I thought she was on her way to weaning. Then DD2 was born and she suddenly wanted to nurse all.the.time again. Seriously, she nurses way more than my exclusively-BF DD2, and she refuses to eat. First thing when she wakes up, she does a 90-minute nursing session (while my tandem infant only takes 15 min), then it's a 2-hour long battle to get her to eat just a bit or two of anything, then she goes back to asking to nurse all the way until her nap, then when she wakes up it's a repeat of the morning routine. I always let her nurse when DD2 nurses so there is no jealousy (and they both love that bond), but she insists on nursing 2-3 times between each of DD2's feedings! It's getting ridiculous. Between the two kids (but mostly because of DD1), I'm stuck sitting down nursing almost all day, then my co-sleeping DD2 nurses throughout the night. I love tandem nursing and extended nursing, but this round-the-clock feeding of 2 kids makes me feel like I exist just to BF.
I've tried distracting DD1 but she has a single-track mind. She does know we don't nurse outside the house or when company is over (I was good about setting those boundaries after she was 18 months), so she rarely asks. But when we're at home, especially now that we're buried in snow and stuck indoors, she is very demanding and feels rejected when I gently refuse or try to redirect. I thought that maybe she just wanted affection or attention and was using nursing as an excuse, but that doesn't seem to be the case. Extra cuddles, playtime, etc. just aren't a sufficient replacement for her. :( She doesn't tantrum when she isn't able to nurse; she acts rejected (and it seems genuine as opposed to manipulative) and says she's sad. It breaks my heart.
She's very intelligent, so I've tried explaining that she is old enough to eat food more than nurse, but she's having none of that. (The "big girl" talks aren't getting us anywhere with potty training, either. A related issue, perhaps? She just seems so resistant to any changes that come with getting older!)
Our pediatrician is very pro-extended BFing and keeps encouraging me to continue letting her nurse on demand, saying that she's still thriving in all developmental aspects, it's helped her bond so well with her sister, and by the time she's school-aged, she'll eventually realize that other kids her age don't nurse and stop on her own. Yay, so only 3 more years of this to look forward to?
I certainly don't want to force her to wean if she's not ready, but I sure wouldn't mind if she cut back a bit and ate more food. She is a VERY picky eater, though, so I guess it's better that she gets her nutrition from breastmilk than not at all. It's just so draining for me and so hard to get anything done. She's still nursing as I type, and has been for the past 90 minutes, sigh...