I think that even if a mother were to sacrifice her life to save her children that it is still not "true" altruism. When she sacrifices her life to save her children she ensures that her genes are passed on successfully.

Would the mother do the same for a total stranger?

If she does, then I think there is a reason for it, like the stranger is an important person and if she dies, she gets to experience the good feeling that she has helped society out.