It might sound daft, but if there isn't a willing friend/family member around to help you practice a few questions, what about just talking out loud as a practice run so the questions aren't quite as scary. It doesn't sound as if your DH would be all that amenable to doing a practice run, which is a shame - is there a friend or relative who might be?
I hate being interviewed by people I know, it's worse than when it's strangers! So the way I've always coped with it is to behave as if we are strangers, and they know nothing about me, so that when they're making the decision, they have the evidence to show why I'm the best person for the job. (Also take this approach due to run-ins with HR and recruitment over the years, as both a candidate and an interviewer, but that's a whole separate can of worms!!)
I'm hoping they've at least given you a job description and a person spec, if you look at those they'll hopefully give you some ideas on the type of questions they might ask you - could you prepare some notes and have answers ready, not exactly totally prepared, but at least not totally cold?
If they ask you, "Tell me about yourself", that's often (in my experience) interviewer code for "Why would you like the job and how do your skills and experience fit with what we need?"
You're already doing the job on zero hours, so they know you can do it, and you know you can do it. It's just (!) a case of pulling out the evidence to show that you can.
There are also different styles of interviewing, depending on where you work. Years ago when I joined the workforce it was all pretty hypothetical: "What would you do if a giant purple leopard suddenly raced into reception?" (Okay, that's an exaggeration, but not by much. I did sit through some interviews where they gave me a hypothetical team and wanted to know who I'd chuck out of the hot air balloon to keep it in the air.)
Then in the civil service it was all competency based, for a bit giving examples of previous things you had done, as the theory goes that your past behaviour gives a good clue as to what you'll do again.
And in the NHS now, it's "value based". So you'll often be asked questions about the values of the organisation, which ones you think are most important, and how you can demonstrate them. You're still using past examples a lot of the time, but the focus is slightly different.
The STAR technique might help you to put some answers together as well, one example is at www.thebalancecareers.com/what-is-the-star-interview-response-technique-2061629
For your nerves, if you have a look on Youtube you should also be able to find some ideas for breathing exercises that can help calm and slow you down.
Good luck - you can do this. 
