Parenting advice (older sibling)

Please help, our eldest daughter (3.5) has taken to hitting her sister (1) any chance she can get. She doesn’t hit hard but she is clearly trying to send a message (to me, not the baby as she usually looks at me while hitting) and I don’t know how to handle it. We are not a hitting family and so far I have tried hard to explain to her why hitting isn’t a nice thing to do, I’ve asked how she would feel if she were hit, (she always says she wouldn’t like it), and I’ve also tried sending her to her room but none of these things appear to be working. Can anyone else give some clues as to why this may be happening now, offer some strategies for managing it, or just reassure me that this is normal and ‘this too shall pass’. It is making me (and the baby) so sad. Thanks x