Allocate time to spend time with her. Have someone care for the baby for awhile while you tend to the big sister. She misses her time with you and she deserves that time as well. She just feels neglected and is good that she can express that.
Do always reassure that you love them and show them you care. Try to also have them involved in caring for the baby and use words like “hey, your baby is crying.. what should we do?” That’s if she’s able to comprehend.. avoid using sentences like “Baby need to sleep. You play alone first ok?” Instead say, “Mommy will play with you later, okay? Mommy needs 5-10mins.” The key is to not use the “baby” to sound as an “excuse”