I wouldn't try to force weaning, that could make the resentment and jealousy worse. Feeding is a comfort and connection with you that is important now more than ever during this big transition.

Some of the ways in which we discussed the new baby and feeding was to explain all the ways the milk would help her baby sister (and we always spoke of it as her baby sister to help her bond with the concept more personally, make it someone she was invested in also) grow and how much fun they could have once she got bigger. So there was a huge benefit to her sharing the milk (we also explained the same for sleep so she would know how important to respect babies rest was). I would acknowkedge what she is saying and reassure "it has been your milk and still will be your milk, it will also be your new siblings milk but the more is drunk the more is made and there will always be enough for you both".

We tandem fed for 6 months with no jealousy, I think tandem feeding actually helped a lot. I didn't refuse DD1 any feeds (and it was great for establishing supply and engorgement) so early on after DD2 arrived, she fed very often but it went back to once a day in time (bedtime which was a nice bonding one on one) till she decided to stop. DD1 was 2yr 9mo when DD2 arrived.