If Sierra’s pet mom behaviour isn’t enough to convince you she may be plotting a new career as a criminal, then let’s turn to her motives as a mom-to-be. As we see throughout Money Heist part 4, Sierra is pregnant and on the verge of giving birth at any moment. It is likely she will do whatever it takes to protect her child and their future together, including joining the heist. At this point, The Professor would be smart to accept Sierra into the fold, considering how easily she was able to track him down with just a few scraps of information. Such ingenuity must be useful to a massive criminal organization.