ElChan I've just caught up on your thread, WOW! What a lot has happened. Good for you. I know it's been hard but just imagine if you'd never had that first conversation. Now it's all out in the open, DP has to really think about what's important to him and what he might have to change in order to get that. Saying what you feel is HARD, especially if your'e not a fan of confrontation so really well done (hoping I don't sound too patronising!)
As to what's next. Definitely agree with PP that you need to take a few days alone to do some thinking. I agree with mikesh and others that you shouldn't sacrifice the future you've envisaged with a child of your own. But only you can decide whether that future is possible in your current relationship. FWIW, your DP comes across from your posts (to me, anyway) as someone who is just about keeping it together day to day, who hasn't had the time or energy to stop and think about the future and your needs - he doesn't seem malicious. But that in itself may not be enough - and perhaps it's difficult to know how different he'd be if the strain of DSS was lessened?
You are clearly a very kind and loving person who will habitually put other people first, but do use the time at your mum's to think in a way that you would probably consider 'selfish'
Any factors to do with DP, DSD, DSS and what they need, don't count in your decision making for now. Is this relationship right for you in the long run? Do you think DP will make a good dad to your child? Will he be a good husband to you? What needs to happen before you feel that the two of you are in the right place to start your own family?
Ultimately, only you can decide what's right for you and how much you're prepared to sacrifice. One thing that strikes me from your posts is that doing something that's right for you, even if it has negative consequences for other people, might be a very difficult decision for you in particular. If that's the case, do think about therapy - I have found it very useful in helping me to be a bit kinder to myself and to allow myself to have wants and needs even when these cause disruption to other people.
Big hugs. Keep going. You remain amazing 