It’s very difficult to not believe there is some tie to him when they keep going back to that area near their home. That tells us that there is something authorities are hung up on there or a piece of the timeline that isn’t super clear.
I can say that a missing body isn’t good for a quick inheritance settlement. It usually takes 7 years to deem someone dead without a body. However, I wonder if NG helped them out financially as they were close to her. Meanwhile, the other siblings moved away and do well for themselves. Maybe they needed a loan or to get out of financial trouble and NG declined help. There are so many things to consider, but the fact that SIL was the last to see her alive is enough to consider all potential motives.
Oh and one more thing, does anyone wonder why they would have their mother take an uber to their home only a few miles away?! If that’s the case, why not have her take both ways via uber? If I were to pick one way to drive, it would’ve been the earlier time, not the later. MOO I would never put my elderly mother in an uber to begin with if I lived that close!