I think she just wants the kind of attention her sis is getting, and figures that if she acts like a baby too, then she will get the attention her sis is getting. Believe me, even when you try to give both equal attention they may still want more. Call it jealousy, sibling rivalry or whatever you want they want the attention the other is getting and at times they may want more attention than the other is getting. They don't care what kind of attention it is as long as they are getting the attention. You don't have to give them things for attention, you could just give lots of praise, hugs and kisses, play with them, or even let them both sit with you. Mine like to fight over my lap so I ask them to share my lap, either take turns, or one child on each leg. It even works on the computer, so long as they aren't trying to get in the way of the keys or mouse when I need them.
I have tried to be fairly even handed with my attention to both my children, to be casual with the negative attempts for attention and to let them know the importance of things like sharing, taking turns, helping, having mommy or daddy time, needing alone time etc. Though they occasionally resort to attention grabbers like acting like a baby, they usually do very well together. My oldest also went through her period of reverting back to pooping and peeing her pants (This too shall pass). So I think if you are persistent with giving them both, good loving attention, ignoring or being casual with the "unwanted" attention, involving both of them with as much as possible, and teaching them important things concerning being courteous and compromising with each other they will both probably do fairly well.
It takes a long time and a lot of consistency though, and there will always be times that they may revert back to that unwanted behavior, but it should gradually become less and less. So long as you don't draw attention to the negative behaviors, be consistent (everyone involved should be consistent not just mommy), and give them plenty of attention and mommy/daddy time. Also, keep in mind that it takes the average person at least a month to adjust to a routine or changes, so if you try to adjust what you are doing it may not seem like it works after a week or two (sometime the behavior may even get worse before it gets better), try to give it at least a month to see if you are getting good results from any changes your family makes, and remember be consistent and patient.
I wish you luck,
V.