How do I deal with my two step kids who's mother has been diagnosed with cancer?

I’m 25 years old and have been married for two years. My husband has two wonderful kids who I love so much. Their mother was diagnosed with cancer last year while pregnant from another relationship. She had a hysterectomy and a double mastectomy procedure done in January. A week following her surgery, she was diagnosed with two brain tumors and now the tumors have moved to her liver. She’s very young, only 29. Although we've never had a great relationship, we've come to a point where we need to get along, but we both don’t know how to approach one another. I myself don’t have any bio children of my own, so I don't know what to expect or how to parent these kids and it’s killing me inside to know that my step kids might lose their mother. How do I deal with this? What should I do? I’m devastated.