Ownership of land

Ok, the situation is, my dad was left some land by his father, (which some of his siblings hold a grudge about as it involved changing the will.) He now wants to sell some of this land, so he got an estate agent involved etc. and placed a "for sale" sign at the site. Almost straight away he got a phone call from one of his siblings (who lives beside this site) saying that the piece of land was left to him and that they would have to discuss the situation further. We advised my dad that he had nothing to discuss as it stated in the will the land was left to him, end of story. So we heard nothing more from him for a few months.
My dad had to see his solicitor about something recently - and the solicitor informed my dad that his sibling had been to see him and told him that when he built his house (which is next to the site) he had to lay down a sewerage system there, and if my dad was to sell the land that he should compensate him for this cost as it effects the site as well. Is it just me or is this ridiculous?? The solicitor advised my dad to pay him something, to avoid creating "bad relations", but there aren't exactly good relations in the first place! If my dads sibling had a problem with this site/issues why is he only asking over 20 years later?? Am I right in saying my dad doesn't owe him anything and that if he were to pay something it would be purely just to "keep the peace"?

my only advice is life is too short for this crap - and there is always more to this story - like how is it you know someone changed his will, nobody should have known about what was his the will till your grandfather died so how is it that they did and knew he changed it?
Your dad needs to decide this with the aid of his solicitor who should not be acting for his sibling i would have thought, without (in my opinion) you being a flea in his ear as he has to live with the consequences.

Sorry if it is harsh.

Good point, I'm not sure how my dad knows about the will being changed actually TBH - but I do know it is an issue in the family. As you said there's a lot more to the story. It's just that my dad is one of these people that tends to get walked over, and I don't want him taken for a mug!

so long as he can rest at night what matter if he is taken for a mug?
your grand father might have told this guy that he could have his septic on his land and then relations deteriorated.
Maybe this lad (granddad or uncle?)was always a stroppy sod .Only your father knows really because he would have been brought up with him. Your father got the land for nothing so maybe if he did give him a skelp of the land and still sold the site would it be worthwhile? - its hard to know.
Its better to look a fool than be a fool and if your Dad is happy to offer thsi person some money / land to keep the peace then you should encourage him to do whatever makes him happy in teh long run and bad blood is awful hanging over someone.

What I'm asking really though is, is my father legally obliged to pay for the sew. system or is purely his decision?

Legally i doubt it - say he inherited land from someone he did not know. He would have inherited a problem and would deal with it without feeling guilty. he could go to the council / land registry offices and get the plans of yer mans house to see if indeed he is not winding up your father. Check out the boundry lines/ walls - like how do you get planning if you put your septic in someone elses land?
He needs to do this anyhow as who would want to buy it with this issue looming.
But for your fathers own peace of mind i feel he needs to get his own solicitor (not one acting for both people??) and some more advice. And your job is to support your Dad. Yer man might be a stroppy git or have been promised the land for years before the will was altered, who knows.