I'm so sorry OP, to be honest it sounds like the job is the main source of pressure and anxiety and this is a time to focus all your energy on your beautiful mum, daughter and grandchild.
How realistic is it for you to just walk away from work? What does your financial position look like? What financial commitments do you have to cover. Do you know what benefits you are entitled to?
Where is your mum being cared for? Does she have any assets/income which can help to support you while you are caring for her? are you an only child or do you have other siblings or family members also able to help share the care load?
i would look to simplify your financial commitments, claim all the benefits you can (including any for your mum and daughter), work out a plan for sharing your time between the care and support for your mum/daughter, think about who else is there to support you through all this - look for independent friends / professionals - gp, counsellor, social worker? - who can keep you as their main focus while your focus is naturally on your mum and daughter.
can you get a social care assessment? What support can they offer? Especially if your mum is not already in hospital or a hospice.
look at charities, local support groups, cancer charities (macmillan, Marie curie) and do speak to your GP about any medication and/counselling options.
make sure you are officially recognised by everyone - including yourself - as a carer - you are caring for your mum and daughter right now.
sending 💕💐