Hi there,
I'm sorry that the family is going through the difficult process of possibly losing a loved one. I do hope and pray that your cousin-in-law's mother is able to enjoy life for a little bit longer than anticipated.
While I can appreciate your hurt feelings regarding your cousin-in-law and friend, you have to keep in mind that you introduced them to each other last year, and that obviously they have become good friends. It's OKAY to have mutual friendships, there is no harm in it. I introduced a best friend of mine to another close friend of mine years ago, and they formed a friendship. That was my whole point of introducing them to each other. :)
Your friend ended up sharing with you what she and your cousin-in-law talked about without you asking. That means something, that they're open with you, not keeping things from you. Don't be weirded out by it. This is an innocent thing.
Your cousin-in-law has gained a larger support system, she's going through a really difficult time. You're still her relative, you're still her friend. Just be there for her, there is no need for hurt feelings.
My dad was diagnosed with cancer in July '08. My sister passed away in January '09. I called upon more than just one friend, I needed the support of many people in my life.
Hang in there...
Vida