Penny, you raise an important question with a two-part answer on whether or not you should go see your mother even though she is a toxic person and dying.
On the one hand, you need to take care of yourself.
If she causes you such heartache and your finances are so lean as to not be able to travel 1,200 miles to see her, you should not go.
Take care of yourself first; otherwise there will be nothing left of you.
On the other hand, if you DON'T see her before she dies, you may regret it.
As we grow older and experience life's ups and downs we often gain greater compassion for our differences. Although, this doesn't seem to be true in your mother's case as you describe it, it may be true for you.
Once the blood stops coursing through your mother's veins, she will live only in your memory. You may (or may not) regret that you didn't see her one last time.
If you even think there's a slight chance of this, which I think you are questioning here, find a way to visit her. Maybe your dad and mom can help pay for your trip.
Your and your daughter's futures lie ahead. Your mom has lived her life, made her choices, and although she's not happy, it's her life.
Will I have any regrets if she dies and I didn't see her one last time? If so, find a way to see her.
There are other ways to see her. Consider using a video communication tool like Skype. Most laptops have a camera built in not to mention phones with this feature.
Also talk with your brother. Depending on your relationship and before your mother's death, could be a powerful time to share your feelings as brother and sister.
Another perspective on this can be found here:
Wishing a parent would die