I'm so sorry for your partners predicament. Is his step daughter from a previous relationship? I only ask because often the motivation for such allegations simply relate to the break down of a relationship.
I'm sure your partner knows not to speak with the police without legal representation. You haven't said whether or not he has been charged? I'm certainly no expert, but I can offer you a few nuggets from my limited experience if things do end up in court.
The very best advise I can offer is for your partner to take the time (and there will be plenty of that provided he is not remanded) to go back to the very beginning of that relationship and write out the whole saga; how they met, how he initially got on with the step daughter, all the issues they might have had, examples of past behaviors or lies, what went wrong, how their relationship was immediately prior to the allegations, fallings out etc. Its an effort, but you'll be amazed what comes flooding back once you start, and if the worst case scenario occurs and he does wind up in crown court it will give his defense team valuable insight in to the relationship dynamic and perhaps one or two things to build a case around.
Often in these cases there is no physical evidence and the prosecution will rely heavily on hearsay evidence to prove a case ie. "she told her best friend about it 2 years ago". It is equally important to take the time to read the transcription of the accusers ABE (usually video interview), as well as any written statements that is to be relied on in court; again, there could be discrepancies, inconsistencies, provable lies etc. It is common for the accused to struggle reading this type of content, after all the crimes that are alleged are abhorrent, but DO NOT allow him to put them aside and bury his head in the sand! The video will be played in court anyway so you might as well get to grips with that filth straight away, and even consider introducing the salient points to your supporters so that they know what to expect in court.
obviously it is important not to approach potential witnesses or to be coercive in anyway, but speak with family and friends (real ones) to see if they recall anything you may have forgotten or didn't know, but do not sit on your backside and expect a legal team to start some huge investigation on your partners behalf, it just doesn't happen....if you want justice, be prepared to fight for it yourself, often a defense team only gets to work in the DAYS leading up to a trial!
Make sure you pester your team to ensure full disclosure from the CPS. Once you are sure you have everything (both the used and unused) go through it meticulously, again looking for the same discrepancies, inaccuracies, lies etc. Pay special attention to the unused, there will be things in there that the police would rather you didn't notice, stuff that will undermine their case. Remember the police are not your friend, they are not impartial or unbiased, they and the entire justice system do not care about justice or whether they are sending an innocent man to prison.
If you do end up in court then tell the truth, hold your head high, and don't lose your shit. The jury is watching your every move, let them see the decent person you are and that you don't belong there, after all it is them that will decide your innocence. They are told throughout that they need to be absolutely "certain" of guilt in order to convict.
I sincerely hope it doesn't come to court, best wishes to you.