I think Jeremy was a spoilt brat who got his money far too easily, and didn't want to be a farmer... simple as that. Maybe his parents did succumb to his demands initially for somewhere else to live other than the farm as a sweetener. A move for Sheila into his cottage could even be a reason for the argument that night, with Jeremy being turfed out, having to look for other accommodation. Seems odd that Nevill should have been in such a bad mood on the phone, when according to the family and Barbara Wilson he was normally a placid, easy-going person. I doubt that he lost his temper with Sheila, but could easily do so with Jeremy.
Why? Although I am sure allowances were made for SC's mental illness CP tells in her book how SC felt NB was disappointed in her in that she hadn't made more of her life. I've made my views known on the fact that as far as I am concerned SC should have been removed from the Bambers' care permanently when June became seriously depressed as a result of adopting her. This was never going to bode well for the future.
When you compare the two adopted siblings there's quite a contrast. SC had apparently been expelled from two schools. Although I don't think this was as serious as it sounds. The first school in Sussex it was agreed by all concerned that it was best that SC left due to lack of progress. The second school in Norfolk SC was expelled from on the last day of term for high jinx. Its unclear what qualifications she obtained from school and secretarial college if any. She then appears to have got a job as a hairdresser which she was sacked from. Having then completed a modelling course, which June funded, there's no evidence that SC was assigned any modelling assignments apart from the assignment in Tokyo. Other work included a shop assistant in a shoe shop, waitress, cleaner and glamour modelling but nothing long-term/permanent. At the age of 17 she had an unplanned pregnancy with a farmhand which resulted in a termination. She then had a further unplanned pregnancy with CC and another termination. Her marriage to CC was short lived. She had difficulty parenting her children and from time to time social services were involved. Perhaps her fragile mental state was aggravated further by her use of recreational drugs. Perhaps the final straw for NB was when SC decided she wanted to meet with her birth mother and in fact did so shortly before the tragedy. JB said in his WS that NB and June told SC and JB a few years prior to the tragedy that if they wanted to meet with their birth parents they should contact the adoption agency but I understand that June was very upset that SC did.
I have every sympathy for SC as I feel certain her mental illness and tragic life as above had its roots in her early life ie lots of abrupt changes in primary caregivers and June's mental illness circa 1959. Coupled with perhaps not being a very good 'fit' with the Bambers as in goodness-of-fit theory &%+((£
As far as I can see there's at least as much reason, if not more, for NB getting exasperated with SC compared with JB.