Also, imo, one or both grandparents were then worried about losing all contact with the baby sibling. If an accident happened with Gus that was due to negligence, the grandparents would have not only lost Gus, but the newest grand baby that they've fallen in love with as well if they disclosed what happened to Jess and/or Gus' Dad. They were possibly trying their best to cobble together what is now left of their family.
It would also be the end of the homestead being in their family. I doubt Jess would want to help them and take over if there’d been an avoidable accident. The whole future would be different.