Hugs, this is a very very difficult time and a good cry or breakdown may be just the thing you need to acknowledge the toll this is taking on everyone. In order to make an informed decision on going forward you need information. What exactly will this new aggressive treatment bring to the table. A real chance of remission, or just prolonging life without real quality. How does your father in law feel about treatment and how his care is managing. This is really important. Some people want to go out fighting, others want to be able to enjoy what time they have left, managing their pain, seeing people they love and doing things they enjoy.
I recommend to you speak to the oncology team and/or a social worker where your father in is being treated and start exploring options. None will be perfect, but you will be able to create a plan that will let you feel that you have done the right thing.
My sisters and I did the above when dealing with our parents final illness (not cancer, but still issues on how aggressive to treat), and then based on the information we formulated a hospice care plan for my mother, We were in the process of figuring out my fathers long term care when he started having breathing, and passed away.