Two trains of thought on this based on the fact the current relative seems to have a sentimental attachment to it.
1/ If the relative really honestly wanted to see it back on the road and returned to life, he/she should sell it to the other relative for less than the value for a 30 year old bike that's going to need work. If there's any question on how much that might end up being, ask him to take it to a shop first and get a list of what it needs and then negotiate a price based on that.
2/ Unfortunately, the reality with vehicles is often that the owner has a sentimental value that disregards real world factors such as age, repairs needed, or even the fact they may never ride/drive it again. At that point, there's a very slippery slope to a vehicle ending up costing the buyer far more than if he just walked away and bought something elsewhere.
How about junior agrees to the $4,000 price setting the money aside. Junior uses the money to get the bike safely on the road and gramps get what's left over.
2) I've seen sentiment cost the seller big bucks. Over ask and it sits for years deteriorating and the true value goes down.