When Colum learned that Ellen had disappeared before her wedding in the Outlander: Blood of My Blood season 1 finale, he didn’t try hard to find her. Instead, he focused on figuring out the next step of the plan.
It makes you wonder why he wouldn’t go in search of his sister. Is it possible that the guilt weighed him down? It’s also possible that he just didn’t have the time to do anything.

Ellen didn’t give Colum time to find her in Outlander: Blood of My Blood
When Ellen hid in Castle Leoch, she did it not long before the wedding was scheduled to start. This was likely planned, as she knew it would mean Colum wouldn’t have the time to look for her. However, she didn’t really know how he would react. She also didn’t know how Malcolm Grant and his uncle would react.
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In a way, it was Malcolm’s uncle who decided to not give Colum much time. There was either going to be a wedding, or there was going to be blood. Colum had to make a quick decision, and fortunately, there was an option open to him. It would have wasted too much time trying to find Ellen, who could have been long gone by that point. He had no idea she was somewhere else in the castle.

Colum may have felt some guilt as well
It’s also possible that he felt guilty for forcing Ellen to marry Malcolm and send the assassins after Brian, effectively breaking his promise. Colum should have called off the assassins once he made the promise to her, and the weird moment in the study with the gift highlights that.
Ellen went to give Colum a chance to own up to the broken promise, and he didn’t. That was the moment she decided that she had to leave Castle Leoch, as Harriet Slater told claireandjamie.com in our exclusive interview, and for Colum, it was likely a moment when he realized he could lose his sister for good.
With all of this building up, he may have realized that it wasn’t in his best interest to find Ellen. There was another option open to him with Dougal, and this could offer him a chance to make up for his actions later. He likely even knew that Ellen had run off with Brian, making it clear to him that she knew he broken his promise.
Of course, it’s hard to say what was going on in Colum’s mind in the Outlander: Blood of My Blood season 1 finale. I do think it was part guilt and part just not having time to find Ellen.
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Outlander: Blood of My Blood is available to stream on STARZ.