Looking for some help for a matter regarding land property in Spain.

I live in the UK and have family land to sell in Southern Spain - (my father who is Spanish, is initiating the sale has 50% share, my brother 25% and myself 25%).

I do not have a relationship with either parties and do not trust nor want to give them my Power of Attorney.

My understanding is that for the land to be advertised at the estate agents for sale, it requires consent from all three parties - however I am in the UK and as mentioned do not want to give POA to either of them.

So my question is what do I need to do to go about this? Can I organise a solicitor POA in Spain or do I need to do it from the UK and have a solicitor in Spain sign papers on my behalf?

Also, what/how many documents are needed to be signed to allow the sale?

Many thanks for your advice