what is the percentage of being A rh-...?

Question

...when your parents and siblings are all AB rh+

Answer 1

In your question,
Mom = AB+
Dad = AB+

Possible blood types in children:
AA (25%)
BB (25%)
AB (50%)

Now, for the Rhesus Factor (Rh +/-) it becomes a little more complicated. In your example, we know the parents are Rh+.
This means that the parents could be either of these two:
AB RR
AB Rr

In both cases, they would have AB+ blood.

IF such parents DID have a child that was A- (which your question doesn't state, exactly...?), then we would know that BOTH parents would have to be:
AB Rr

And children would have a 75% chance of being Rh+ and only a 25% chance of being Rh-.

If this is so, than the probabilities could be determined for individual children as follows:
AA+ (75% of 25%) = 18.75%
AA- (25% of 25%) = 6.25%
BB+ (75% of 25%) = 18.75%
BB- (25% of 25%) = 6.25%
AB+ (75% of 50%) = 37.5%
AB- (25% of 50%) = 12.5%

Thus, the chances of A- is 6.25%.

However, if you are not saying that a child has A-, then it might be impossible to have a A- child. This is because one of the parents might be AB RR. If EITHER parent is AB RR, then there is a 0% chance of a child being AA rr (A-).

Here is a little overview explaining blood types, etc:
http://www.drgreene.com/21_59.html