Christ, if you want a relationship with your daughter (and future grandchild!) do not do anything @LollyLol suggests.
My DDs are small but I have thought about if this were to happen in the future, before they are really adults. It’s a potential scenario when you become a parent unfortunately, no matter how much you talk to them, accidents happen.
There’s nothing more you can say now without pressuring her into an abortion, which you already know is unfair (and will ruin your relationship with her anyway) Explain to her you will be supportive, but not parent the baby. Decide now on how far that support stretches (financially and practically in terms of childcare) and explain that to her.
It’s good that you’re letting her stay, kicking her out would make life harder for her and long-term more likely to increase the risk of SS involvement and you having to step in.
Ask her how she plans on affording baby items now, she needs a part-time job now at the very least, look into what financial support she will get once the babies born (mat allowance, child benefit, UC) and help her work out a plan (she’s unlikely to really factor in all costs, so how will she afford all baby equipment, talk to her about breastfeeding to reduce costs).
You keep letting her boyfriend off the hook here, I would be furious that hes basically shrugging his shoulders and saying he’s ‘basically not going to be involved’ and would be contacting his parents. He had sex with your daughter knowing that this risk may happen, even if she said she would abort if it did, it’s still a potential possibility that she wouldn’t, or would find out too late. We need to be teaching boys that sex=potential baby, regardless of what is said, and that they need to consider that before they have sex! Teenage fathers are always let off the hook and it’s the teenage mothers who deal with it all and face the backlash.