We live in UK

My father lives in a property which is registered as being owned by himself, me, my brother and sister.

My father wants to sel the lease and divided the proceeds amongst us. My brother and I agree.

Can my sister block the sale?

She could but a good real estate attorney can get a judge to force the sale. Ask your sister if she'd buy you all out or try to sell your share. On the other hand who'd want to own with your sister as a partner?