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I'm gonna go with D... Here is my reasoning:

I am assuming its hemophillia A because:
sister is not affected (more likely X-linked)
hemoarthorsis is present

If that is the case... the patient's mother must be a carrier (father cannot pass down to affected son)
The patient's mother has a 50% chance of passing the mutated X chromsome to the patient's sister
Now if the sister has a daugther... there is 50% chance that she will give her defected gene but this will be masked (unless unfavourable lyonisation occurs). If the sister has a son...ther is a 50% chance that she will giver defected X chrmosome and since the father gives a Y... the son will have hemophillia.
So we are dealing with two dependent events: the likely hood of having an affected child (1/4) if she is a carrier (1/2) = 1/8
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