Firstly welcome to Nameberry!
I don't think you have to go with a K name unless you want to, just choose a name you fall in love with. If you do go with another K name just be aware that it gives off a bit of a Kardashian vibe.
I'm not sure of the what style you are going for but here are some name ideas,
I hope that's helpful! :)
I agree: sticking to the same first sound, esp. a hard C/K, sounds a bit Kardashian to me. Most of the time, parents' first names aren't said in the same sentence with their kids, so I don't think a sister would feel left out if she got a different first initial.
With Colt, I'd do:
Willow - seconded
And in case you really do want to go with another C/K name:
I think now is kind of your last chance to leave the K thing behind. My husband and I both have first names that start with a K. We will purposely not name a LO with a K due to the matchiness and the K-K-K comments from all our smarta$$ friends. (We live in Mississippi; it would just be too much LOL.) However if I loved a C name with a K sound I'd use it. I'm surprised my husband hasn't suggested Colt, actually, as he is a huge fan of Colt firearms.
I have a step-cousin named Colt. He and his sister Jacy are junior rodeo campions. Though it's nms, I like the name Colt. It sounds like an energetic, trustworthy, salt-of-the-earth person. Here are my suggestions for a sister:
With a K sound: