J.F.
Please remember that she wll always need her mama- You feel like you are losing her but you must realize it is just because things are difficult for both of you right now and she loves you very much! You are not alone and will get through this phase :) My advice is to , of course, try to make a special time every day to spend just the two of you, but also- You could try to involve her with your care of the baby as well. If she feels needed and appreciated and has a purpose, it could help her feel more secure as a part of your growing family. For example, give her little jobs like handing you clean diapers during changes, holding baby while you read to them, etc...
Another idea is to give her a special activity to do only when you are unavailable to her because of the baby- for example, a basket of art supplies, toys or whatever that she only gets to play with when you are feeding the baby. This should give her a positive feeling about you and her little sister spending time together & make her feel included. Hope this helps! Good luck