Exactly, it isn't just a case of someone you may not like inheriting a lot of money and part of a business you have also investments in - you would have to interact with someone you believed to be a murderer. Someone capable of killing his own family including two children. If you knew he wasn't keen on you, would you not have cause for concern?
It's a shame that there are two issues going on here. There's the major one of the murders but there's also the issue of the probability, that regarding the business, Ann and Jeremy were singing from very different hymn sheets. I imagine, if Jeremy hadn't sold it over their heads, Ann would have been the one doing all the work whilst Jeremy creamed off the profits. I've always defended the rights of adopted children to inherit in the same way as biological children, but I can see very clearly, in this case, Jeremy inheriting COULD have seen him destroy the life's hard work of the entire family.