You need to get them round and have a proper conversation about this. Your DH needs to be a part of it too, he's certainly not unreasonable to refuse to go along with this cheeky fuckery, but if he wants you to say 'no' he should be there to support and back you in doing so. You're supposed to be a partnership and this affects both of you.
Firstly, I would ask them what has prompted this and whether they are in trouble financially. Have they got themselves into debt? Has one of them already quit or lost their job? Are they in rent arrears? It seems such a half-baked, poorly thought out plan that I can't help but wonder if there's more to it. That could just be me being cynical though!
Once you've established there's nothing more troubling behind this, tell them that you think it's admirable they want to save for their own home but you're worried they haven't thought this through. One half of a couple quitting their job with no job to go to is a risky move, both of them doing this at the same time is just reckless. I would also tell them that you're surprised and a little hurt that they didn't even think to ask you how you felt about them moving into your home along with all their pets, and have even told GF's family before discussing it with you! Assuming your DS and his GF are aware of yours and DH's health issues, I would also express surprise that they haven't taken this into consideration. They don't seem to have considered the impact on you at all. Explain that you love them and want to help, but what they are asking (or in fact, assuming) is too much. Tell them that rather than saving money on rent but paying out for storage, they are welcome to store their stuff in your garage at no cost so they can rent a smaller place/a room in a shared house to save money.
In your shoes, I would probably offer to give them some money towards a deposit but obviously you may not be in a position to do that, and you're certainly under no obligation to.
I think you're going to have to prepare yourself for a difficult conversation. If your DS and his GF are they type to assume they can move into your home without even asking then you're probably going to have to be very firm.