Okay, to clarify - "willing" the land isn't the correct term and is confusing everyone. You meant they gave it to you so it is legally yours and the sibling.
This is what I get from reading the thread:
You and sister outright own land.
Was pretty much worthless and you didn't have any real plans to do much with it?
Your dad can go hang out there as long as he's alive, but has no legal rights to change anything on it - just visiting/living on the property if he so chooses.
Gas company ran lines and are offering YOU & SIBLING AS LEGAL OWNERS OF THE PROPERTY a (one time or more time or what?) payment of $433,000.00? This is a done deal, right? It's yours and sibling's money then. Your land, your money. No one can swoop in and take it if you have the ownership and all the legal stuff in your name. Unless you just want to hand over the money... I wouldn't.
You say your inheritance is shot? Why? You had no real plans for the land anyway right? SO a rural cabin off in the woods, even with a permanent easement isn't that big of a deal as long as the gas company paid for the easement and maintains it and maybe give you a bit of money each year to access their easement (I have no idea how that works). So therefore this otherwise worthless land means you are now receiving an "inheritance" of almost half a million to split with a sibling? That's pretty amazing, actually.
The grandparents most likely can be ignored, if you wish. Obviously you should go see a lawyer and ask them if the grandparents have any legal standing to demand for you to sign over the land to them, but likely they'll say hell no, okay you and sister to tell them to go pound sand. Which is kind of what I'd do. Either ignore them completely, or have a lawyer draft a letter to tell them to kindly go fuck themselves in legalese.
Or like brute said, tell them they can buy the land back for whatever it is worth now. If you don't want to deal with it later or the gas line somehow ruined the view or something, you can sell it to ANYONE, just offer it to the grandparents at market rate first if you really want to (but you don't have to unless there is something specifically in legal terms that prevents this). But you and sibling still keep the gas line payout.
So TL/DR:
Your land? Then, your money. Confirm everything with lawyer. Do what you (and sibling) want once confirmed of your legal standing.