We know why this happen, it's jealousy. She is now keen on doing what DiDi does to get attention. Mummy also try to spend as much time as possible with the "big one" together with the "small one".
Up till today, she is much more better than before and never cry or scream when DiDi is crying. But, only if DiDi's "speaker" too loud, she will continue with hers too. We still need time to overcome the sibling rivalry... :(