Hello OP,
First off, you might find it helpful to ask MN to move this thread to the SN chat board, as you'll get posters there with experience of these specific issues.
Secondly, does your sister have a diagnosis of autism herself, or is it suspected? I'm so sorry to hear she tried to end her life, that is incredibly sad.
As you may know, MH issues are frequently co-occurring with autism, particularly in undiagnosed or late diagnosed women.
Do you mind if I ask who in your family is already diagnosed with ASC and/or ADHD? Is it your sister's side of the family, her partner, or both?
Recognising autism at age two is perfectly possible but it will depend greatly on what the child's presenting differences are, and of course on what other environmental or medical factors could be influencing differences. (I get that you know this, and you've been up front that your sister's MH is not good and she needs support, so I'm not having a dig - absolutely none of us set out to do less than our absolute best as parents, and sometimes we have tough things to deal with.)
Can you list what differences you observe in your nephew and what makes you think these are autism related? You mentioned speech delay and motor, can you expand a bit?
It's worth looking up Cathie Long, an independent social worker who specialises in independent assessments for the court where autism in either the child or parent (or both) is suspected. She may not have capacity to take new clients as she is currently doing work around undiagnosed autistic mothers being accused of FII, but it's definitely worth reading up around her work and contacting her to see if she can advise.