It may not be a direct death threat, but it is unacceptably over the top.
Yes it is over the top.
No it is not a death threat.
We all know the jw lingo. She is not threatening their lives. She is telling them they are going to die at Armageddon. She is trying to scare her brother back in.
So, if they are putting up Christmas decorations and they don't mind being seen with that, why can't they just end it all by being up front with her and telling her in no uncertain terms that they - He- is not returning and to stop doing what she is doing?