Lack of Family Support

Hi everyone,
​My Husband and I are TTC #1 for us. I have a DS from a previous relationship and he has a DD from a previous relationship.
​Over the last few days, it's really dawned on me that were we to have a baby, we'd be really on our own with it.
​What I mean by this is that:
​A. I don't get on with his Mum and his twin Sister.
​B. His parents are the oddest grandparents I know. They don't get involved with their grandchildren (my DH's Sister has two young children also). His Mum moans if she has to look after them etc.
​C. When I had my DS, my Mum was amazing - she was there to look after him for an hour if I had had no sleep, or she would ask to have him for a weekend because she loved to have him! Unfortunately, she passed away two years ago :(.
​D. If I am ill, or if I am stuck for childcare etc, my DS's grandparents will happily watch my DS (he is 8). They ADORE him and are always asking to take him here and there, on holidays etc.
​If my DH and I were to have a baby, I don't know if it would be difficult having no support for those days when you're ridiculously tired, stressed, emotional etc.
​I know this is a bit of a vague post, but just wondering what you all thought. Am I over-thinking it all?
​I think if I hadn't had children already and wasn't  aware of the tiredness etc, I wouldn't even be questioning it!
​Much love ❤️ xx