Unfortunately, the pressing issue here's these two lovebirds know a huge amount of people. Rick has most of his writer critics and friends, including some he doesn't even like but nonetheless feels obligated to invite. Meanwhile, Beckett has her family, and most of her associates at the precinct.

Where do Stana Nathan and Katic Fillion’s characters turn to for advice? Think such as Martha. Rick’s mom is really a sensible woman known to make big decisions … right? Well, unfortunately for each of them, the only thing that Martha really seems to do in the sneak peek below is introduce more potential names to the table. They literally have hundreds upon hundreds of people to account for, and that is so many to keep track of.
If someone's surely got to maintain danger, It really is wanted by me to be Castle. It is not only exciting because he's the main character, but it's dramatic for Beckett as well because he's her fiance. Plus, his daughter and mother would worry about him too, which adds to the drama and gives both of those characters something interesting to do.
And it could make sense that if Beckett pushes the wrong buttons in her investigation of Bracken that eventually his people are going to push back -- getting her emotionally riled up is a good way to throw her off and get her off guard, so having Bracken's people mess with Castle would be a way to do that. They'd be going after someone who has an important place in her life and heart.
The writers haven't really been following through very well. They had a golden opportunity when Castle was poisoned, but instead the episode fell flat because of the way it was written, especially with how they didn't show very well the impact that kind of situation would have on both Castle and Beckett, and Castle's family. It was touched on but not well.