I'm pregnant and my mom is terminally ill

Myself and my fiance have been trying to conceive for a couple of months now, we have not long brought our first home and we felt ready to start a family. A few months into TTC we found out that my moms cancer had returned! She had breast cancer 4 years ago which resulted in her having a double mastectomy... she recovered very well and was soon approaching 5 years cancer free, until now. Her breast cancer has returned but has travelled to her kidney and lungs. Due to it effecting so many organs it is classed as a vicious cancer which is not curable. OMG! Emotions are all over the place. Me and the fiance had a conversation about whether we should carry on TTC, but agreed that there would be no perfect time in relation to moms situation, so we carried on trying. During the meeting with my moms oncologist, he said that she will have a couple of months, maybe years left to live but certainly not decades, they will give her chemotherapy to relieve symptoms, but it won't cure her. I was and still am heartbroken, confused and angry. I remember waking up in the middle of the night and crying for ages upstairs asking myself, why would you ever want to be a parent... bring a child into this cruel world, to leave them one day! It's horrible! My nan is also suffering from dementia very badly and has been admitted into a mental hospital, so watching my mom go through this aswell has been truly horrific. So the thought of having a child and them leaving you one day or vice versa seemed like a cruel idea, however that thought has slowly vanished but it will never leave me. My mom is my best friend and having a woman like that raise me has been a blessing. Now... about a month into my moms diagnosis I found out I was pregnant... exciting times, kind of! I am around 4 weeks now and my moms first chemotherapy session is due in 2 weeks. What do I do? Do I tell her now? wait till my first scan to let her know... in case baby isn't well? I am confused. Also, how do I cope with the stress and worry about my mother and carrying a baby. What would you do?