Who kills Sigma Phi, and Diana?

It is Mira. There's a few ways to tell. At the end of the route when Diana goes to key in the passwords to end the game the announcer declares 6 participants dead (Carlos, Junpei, Akane, Eric, Sigma and Phi) the names missing are Diana, Q and Mira. While Q could be the killer further information will strongly implicate Mira.

There's the distinct clacking of heels as the killer walks which fits none of Diana, Phi nor Akane who are all wearing flats (none of them make the clacking as they walk). Its somewhat difficult to make out but the killer also has fake nails when they show up in front of Sigma and Diana which is another trait unique to Mira.

The above can be gleaned from the immediate story surrounding the murders and below is additional context from elsewhere in the game:

Plotwise, this is the timeline where C team is executed by their collars and Q team wakes up in the Bio Lab and chose to inject themselves with the Radical-6 Virus (i.e. not the timeline which leads to the events of Virtue's Last Reward). Afterwards while Q team is chilling Mira shanks Eric with a scalpel and then stabs Sean. This is distinctly the timeline where D team escapes the healing room because they are shown Eric's body splayed out after he was stabbed.

Another note is that Mira is the resident serial killer in the 10 participants of the Decision Game, having racked up 18 kills prior to its start as the infamous Heart Ripper. She kills Eric and Sean across many different timelines and is also responsible for killing Junpei in the timeline where D team is executed