I think 1:300 for a 40 yr old is a really good number, as this means if your sister had 300 babies then 1 may possibly have DS not 1 baby out of every 300 babies born in the general population. And I feel pretty confident your sister won't be having 300 babies

As everyone has said the decision for an amnio is your sisters and if the baby did have DS would it change her mind at all.