Ownership and the law in the UK?

Hello
I don´t know if I worded my question correctly.
Would my sister be entitled to own her house if its just my brother-in-law´s name on the deeds.
She has no reason not to be on the deeds but he pays the mortgage. He told me this not long ago and I was a bit shocked. It actually came out accidentally and he was proud to say it. They owned a house before and it was the same thing.

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