Both my parents had brain tumor and brain cancer. My mom had surgery to remove the tumor from her brain in the early 1970s. Gathering all the tumors that were spread all over her brain, the size ended up to be like a grapefruit. During the surgery, she lost vision in one of her eyes and sense of taste. About ten years later, the tumor returned. She had another surgery, and that time the size was like a pea. More years passed, and it returned. She opted not to have a surgery. She felt she was too old for that, and she wanted to die anyway. Ironically, she lived to be 88 years old and passed away last December from natural causes. Whereas with my dad, in 1995, my parents' friend from Canada paid them a visit and since she had not seen them for a long time, she immediately knew something was wrong with my dad. At her suggestion, my dad saw his doctor and was diagnosed with brain cancer. It was quite a shock for us all. The doctors recommended only radiation treatments due to his age (he was 78) and emphysema (spell?). He lived only seven months and died seven weeks before my youngest son was born. If he had not received radiation treatments, he would have been gone in three months, but the treatments bought him four more months.
You just never know how long your MIL's brain cancer will be. It can be quick or it may take a long time. It depends on what kind of brain cancer she has. There are various brain cancers. You will have to do a lot of research on this matter like my sister did on my dad. Talk to your MIL's doctor and ask what kind of cancer she has. If her doctors are optimistic, it may be good news then. However, you can insist that they be honest with you.
Good luck and I hope your MIL is in remission and don't get it back again!