Lomond I completely understand where you are coming from and how you feel. I feel those who go on about how you will feel when the baby is born are completely barking up the wrong tree. The question of how you feel about the baby when it is born has got nothing to do with your anger with son and gf, who have just conceived a child and have made no plans about how they will manage after the birth, beyond sponging on their parents.
I think they are even more selfish in that they have clearly not wasted a single seconds thought on how this will effect his younger sister who has a diagnosis of autism.
I think these two need to be given a big wake-up call. You need to sit them down and tell them that they cannot live with you. Your responsibilities at work, that your youngest child is autistic, will be more than you could cope with, and if you have a mental breakdown, will they look after you?
The bit that no one is focussing on is that, although you had a baby very young. You did everything you could to stand on your own two feet and support yourself - and did so quite successfully.
Tell them that if their are mature enough to have a child, they are mature enough to make plans for how they are going to support themselves and their child. They need to find somewhere to live, need to plan for the drop in income while she is on maternity leave and need to plan how they will manage about childcare when the gf goes back to work.
I am willing to bet that their ideas for childcare are to assume that their parents will provide it. That you will not, I assume, needs to be said now.
However, one niggle, Your son has clearly grown up with an inflated sense of entitlement, that he can do what he likes how he likes and someone will provide for him. He must have learned that at home. I do think that you need to ask yourself, how far you contributed to him expecting you to subsidise him and perhaps to begin with you could offer them some help with the rent, if they get their own accommodation, not all the rent but some of it and phase it out over two or three years.
Either way you need to make it absolutely clear to your son, that his assumptions of how you are going to support him and his family after the child is born are doomed to disappointment
And when your grandchild is born, you will probably love them, but that is entirely irrelevant to the problem you now have. I would also add, that my DH had aunts who were younger than him and a friend with a large family was pregnant with her eighth child while her daughter was expecting her first. The two boys, now grown up found it hilarious that they were uncle and nephew and could always be guaranteed to misbehave in public shouting 'I am his uncle, he is my nephew' and falling about laughing.
