Please help me

[holeymoleyinaholey]

I'll try to be concise...

My Dad passed away leaving no will. He and his partner were not married and had a child. So myself, my two siblings and our half sibling will inherit a 25% share of the estate.
The difficulty is our Dad paid for a property but it went into his partners name only (we do not know why). Our half sibling will now inherit this from their mother. The property is worth at least 5x what our Dad's estate consists of.
Our half sibling is not willing to give up their share of the estate even on reflection of what they will be gaining from their mother thanks to our Dad.

We strongly disagree with them taking a quarter of our family home that they never even stepped foot in.

Is there anything we can do?

Can't see what. Presumably partner is still alive? Maybe your Dad put the house in her name so she was provided for as well as the kids,

You don't know half sib will inherit from their mother. It may go in care home fees, to the cats home, on a world cruise or to another man if she marries again....

It can be argued your father had an equitable interest in the other property. The executors can issue proceedings in court for a declaration to that effect, but they would need to take some detailed advice before embarking on this course of action. It all depends on how the evidence stacks up,