Hi Pennine,
Have been reading this thread with interest.
Whatever happened regarding your "sister out of the blue"? Did you eventually make contact and/or meet up?
You mention 2 more sisters and 2 more brothers. You also mention a step-sister in Australia who knows nothing about the others. How did these "new" siblings find you?
Unfortunately, it's hard to give advice because each person's story regarding half-sisters/brothers and step-sisters/brothers is always unique and obviously very different.
I guess it also depends on the fact that none of you really knew about each other until someone started to ask questions? (You mentioned that you thought you were an only child).
From my own experience I found out that my father had 7 more children to another woman. I and my 3 younger brothers were incarcerated in an Orphanage while my father saw fit to have 7 more children and play "happy families".
To cut a long story short, I never acknowledged these children and never will. A lot of water flowed under this bridge to make me feel that way. In short, I resented them.
On the other hand, my mother remarried and had 2 more children who were given up for adoption. I would do anything in the world to find them as there is no resentment at all. Just a loving urge.
My point is - there are two different sets of half-siblings here and I'm only interested in my mother's last 2 children. Why? If I found them, I would be extremely cautious and I would take things slowly. Personally, I would make a few phone calls and only then would I agree to a meeting. You can guage a lot about someone after a few phone calls.
You have to remember that some (not all) siblings harbour resentment and many other feelings, especially when they "think" or "assume" that one sibling had a better life than them, etc. etc.
On the other hand, some siblings are just so excited when they find they have new brothers or sisters. I have heard of many wonderful, positive stories. Let's hope that yours is a wonderful, positive experience.
The only advice I would give you is to make phone contact first - make a number of phone calls before you decide to meet. Hopefully they will also phone you. Remember that it can be quite daunting for both of you.
Don't give them your home address or home phone number until you are certain that you trust their motives. This could take a while. Treat them like acquaintances who have approached YOU. They aren't your friends YET, either. You need to establish that. Maybe a good idea to give them your cell/mobile phone number and see how many times they phone you. You can always get a new cell/mobile phone number if things don't work out.
You also have the option of walking away and not having any further contact and you don't have to justify yourself to anyone.
Good luck Pennine.

Big hugs
CP