Unless you adopt her, you shouldn't be legally responsible. Her mother would be or a blood relative.

Nope.

No her other parent is legally responsible for her. Marriage isn't adoption. Thank God.
His estate may still have to pay CS. That will depend on where you live.

A little off the topic of being responsible but I think still relative, and what a lot of people don't think about..or know.

And I seen this happen to a dear friend of mine.
So let's set the scene,

Your married and have minor skids, in this scenario or most, it DOES NOT matter what the custody agreements are, a horrible thing happens and your spouse passes away., WITHOUT a will, your skid or rather their representative (usually BM) can and most likely will take half of your EVERYTHING! The only thing exempt would be life insurance policy and that's only if they are not named as a beneficiary.
Seen this happen, could you imagine going through the loss of the love of your life, and all of a sudden being slapped with a court order for half of your savings, Equity of your home, his personal affects, vehicles etc.. ?

After we watched speechless at that, happen to our friend, my DH and I immediately went to an estate attorney to protect ourselves bc his baby mama would do the exact same thing to me while probably smiling the entire time.
The law in our state, says as long as there is a will in place willing something (even just a dollar) then the estate can not be touched by minors or their representatives. Don't get me wrong, the option to contest the will is always there, but not guaranteed a win.
My DH makes a great deal of money, and we are financial comfortable, but with that being said, we have both worked extremely hard, and saraficed a lot to get to this point. I will be damned if a worthless white trash baby mama is going to swoop in and take that away from me and the other children. So we are now protected, to sum it up, our wills ( yes we both have them and also have a joint will, in the case something happens to both of us) states, skid gets a sum of $1,000 and some personal effects ( which are listed specifically in will). Our life insurance polices are also only in our names for beneficiaries, so those are not in jeopardy of being touched. In the event something happens to both of us, we have named family members as beneficiaries that we trust to take care of our children. Now, let me be clear, we are NOT trying to cut out skid from inheritance, it's the BM. She would squander any monetary gain of skid, and eventually he would be left with nothing, DH and I have an agreement, we will follow through with the $1,000 granted in the will, but the surviving parent will make sure that the child is taken care of. For example. If something ( God forbid) happens to my DH while skid is still a minor, I will continue to pay for his private schooling, his uniforms, and I would continue to pay BM child support ( although I wiould decrease the amount ) as child support can not be court ordered after death in our state, and BM would be able to fill for my DHs social security death benefits and would gain money each month from that. I would most likely never see that skid again ( which would be fine by me) but I would stay in contact with his maternal grandparents and make sure his needs were met. I would also follow through with the extras we give our children like a car a 16, bday and other holiday gifting. And a trust fund would be set up for college or adult hood use. My DH and I also both have grown children as well as minor children our wills are set up differently for the adult kids.

I know it's an awful thought, but Step parents need to protect themselves !! Being in a step relationship means there's always an outside person. When death comes so do vultures usually in the form of BMs. Just some food for thought.
If you don't already have something in place to protect you, I highly highly ( can't say that enough) recommend you get it.

Only if you want to and if you fight for custody or guardianship.

IMHO of course.