Having DC did my autistic DP in. Both DC are autistic, now teens, although it presents differently in them and their dad. He always struggled with anxiety around routines, unpredictability and transitions, but since becoming a parent, he has experienced some very challenging MH problems as a direct result of feeling completely overwhelmed and like he's not coping (he is very conscientious and likes to do things 'right' in every aspect of his life, and finds the flexibility, nuance and relentlessness of parenting entirely outwith his capacity). I wouldn't wish how he has experienced parenthood on my worst enemy, although I now regularly feel so let down by him that our relationship is just limping along, while I try to care for the DC and stop him from constantly spiralling with overwhelm which manifests in intense anxiety, intrusive thoughts and not infrequently, self-harm. Parenthood is, by definition, all the things he cannot cope with as an autistic person: despite being well-meaning, he's trapped in a cycle of good intention, crippling expectation, overwhelm, collapse and self-loathing.
Really, really think this through, OP. Get all services you can involved at the earliest opportunity. You are so used to supporting your DD that you most likely do not even realise the extent to which she relies on you.
I work with children and families: one very basic indicator of childhood wellbeing is a child's sense of absolute certainty that it can rely on and trust its parent to put them and their needs front and centre of their lives, and to have the maturity to, as necessary, subordinate their own wants, wishes and needs to those of their child. Will this be the case with your grandchild?