M.B.
She is needing some special time with you, also. I know it is hard (my first 3 were born within 4 years... August '82, April '84, August '86).... I had to deal with some of that, also. I don't remember all that I did, though.
When the baby is asleep, make a point to do something with her.... you may be trying this already. The cleaning can wait... don't feel like you have to get everything done when the baby is asleep....
One thing, rather than punishing the misbehavior, start really noticing when she is playing nicely, or behaving nicely, and praise her bunches for that... she will prefer the GOOD attention, and not be seeking ANY kind of attention, even negative attention.
One thing that some people have done is make the nursing time also a special time to sit with the toddler and read a book, or the two of you sit and watch a special show together...... this is giving her time, also.
Does she have a baby doll SHE can feed while you are feeding her brother? My daughters did that, also.... but they didn't want to use a bottle... they pretended to nurse the baby, also. Teach her how to take care of her baby while you are taking care of your baby.
Good luck! I know it is tough!