Firstly, Congratulations, I hope your pregnancy is going well and that you're feeling ok. I haven't read every single response on here but I wanted to put my ten pence in cause I had a similar situation not that long ago and now I have hindsight, I think I can help.
I was 19 when I fell pregnant and only six years prior had I watched my older sister suffer a still birth and I was the only one who took care of her, I was 13. What my sister went through, seeing my niece and burying her, taking care of my sister in the aftermath was extremely traumatic and is part of my PTSD. I am pro-choice but I don't see myself as having a choice because I know full well that my mother and sister are against abortion and loosing my niece heavily impacted my decision to keep my son. I often say that I had him for my sister, not for me, not literally because I raise him but I had him in honour of the daughter she lost. So, in essence re: your aunt, I understand where you are coming from 100%.
However, in hindsight, which is a wonderful thing, I agree with a few posters who have said that your aunt's predicament should not impact your decision. It is honourable, yes, but you cannot take the baby back, there is no refund. You have to be sure. You have to take your aunt out of your decision making process.
If I had truly thought it all through, even then I knew, I would have considered it more than I did at the time. I was stuck in an abusive relationship that was only seven months in. I put up with him for our son for several years later but eventually I gave him back his ring and I left. He is still driving me up the wall now, I am stuck with him, even though we aren't together, we still have to deal with one another for our son. Its volatile, exhausting and controlling. If your partner is any of these things, I urge you to think about it carefully. It's not just you that it will affect, its your baby and everyone around you. My nine year old gets upset that his dad takes money and doesn't pay it back, that he wont pay maintenance, that he breaks promises all of time. Kids are smart and you cannot hide it.
RE: Grandmother. It's harsh and unfair but her extreme reaction actually might be because she cares about you. Now, an ultimatum is not the right way to show it. At the end of the day, this is your body, your choice, not your aunt's, not your Grandmother's. Choose wisely.
An alternative, it is hard but you can do it, is to be a single parent if it all goes wrong even before the baby is born or after. It sucks but you can do it. I would also advise to do everything by the book, put him through CSA, go through mediation and the courts, otherwise you are up poo creek without a paddle, like me!