Talk to a specialist agri tax advisor. Make sure you get recommendations from people satisfied with the service though, as opposed to simply flicking through golden pages.
AFAIK, there was a change in law in JAN 2015, where the person getting the land and relied has to dedicate a certain amount of time farming per week. This was a huge change from previous law. I also understand, but am open to correction, hence the advice in first sentence, that the person who got the land, can GIFT, free of charge a section to another, without the person losing the relief. If that is so, then under no circumstances can you pay mother for the site.
Get professional advice, its the cheapest in the long run.