It may well be that there had been a conversation about the boys' care -although such was embellished by JB for his own purposes and he may even have spiced it up. Just because the Bambers supported them financially, they had no legal rights over them.
The Bambers also both supported Shelia and Jeremy. Both were adults who were able to choose their own path. But they couldn't. Money talks.
Although 28 and 24, they both were essentially children. Both had not managed to make a life for themselves. And both were heavily reliant on the their parents finances. They were not independent. Both lived in properties owned by the Bambers. But accept Shelia was ill and Jeremy did work on the farm.
It's my belief that as good of people Nevill and June were, they managed to steer the children in the direction they wanted them to take because both knew if the didn't step into line the financial lifeline would be at risk. They used money as a tool in doing this.
With this in mind I believe Shelia would do whatever June wanted. Even if there was Colin to deal with as well.