Is it probable? Not remotely probable, given the weight of the evidence that it was her baby, IMO.
I don't think anyone really has suggested that or thinks that. From everything I've read, I think that she started taking care of Zero when he was around three and he came to HI from where ever he was living before that. I don't think anyone saw Stanley Ann with a baby in Seattle in 1961.
To clarify:
I think some people (or one or two) have claimed they say SA with a baby, but I think they were coached and are lying, or they saw Anna Obama with a baby. I am convinced that Anna Obama and Stanley Ann Dunham are two different people.