How likely is it that a person's progeny has an inheritable disorder based on this pedigree?
This is a very rough draft about the case, not verified, and should not be used for any medical conditions. Ask your own doctor. It is just for demonstrating some Mathematics and genetic passing generally.

Condition: Muscular atrophy
Differential conditions: TODO
Support: history of genetic passing in -1 and -3 generations

Disease

Muscular atrophy as a condition jumps over a generation. The environment/living of your greatgreatgrandfathers determine your grandfathers future, as a rough estimate. 
The disease's common feature is recessive autosomality. 
Assumptions


Autosomal recessive disease because everyone who exhibits symptoms is not be able to reproduce due to their early death.
TODO genetic passing
What is the genetic profile? Is there any other autosomal recessive diseases in the gene map?


Proposed probabilities where the random variables $X = disease$ and $Y = carrier$ such that the subscript 0, -1, -2 and -3 is about new children's, John's, parents' and grandparents' status, respectively. 
Both of John's grandparents must have been carriers 
\begin{equation}
P(Y_{-3}) = 100\%
\end{equation} 

The probability of John's dad and any aunts and uncles having the disorder is (TODO think this again with the correct genetic passing)
\begin{equation}
P(X_{-2}) = 25\%
\end{equation}

The probability of being a carrier is for dad, aunts and uncles (TODO think this gain with the appropriate genetic passing)
\begin{equation}
P(Y_{-1}) = 50\%
\end{equation} 

Disease as a process


$X \sim P_{i}(\mu, \sigma)$, $i = -3, -1, 1, ...$  where I propose the mean $\mu$ and the variance $\sigma^2$. 


To understand this better, we should know how the generation $-5$ lived. 
Can you get it?

Assumptions


Assume the events of individual probabilities independent i.e. (-3) events do not affect (-1) events in the series. 
...


Test if the two independent statistics $X_{-3}$ and $X_{-1}$ are from the same population. 
If you assume it, put it to assumptions. 

Probability Density Functions in Generations -3, -1, 1, ...

Probability density function


At $X_{-3}$, the 3 aunts of 4 died. The mean $m = 0.75$ (=3/4). 
You cannot use Binomial because sampling is not with replacement, not Poisson because occurrence probability is not less than 10%, ...
...


Conditional probability and Bayes theorem


Consider the conditional probability $P(X | Y)$ that is what is the probability of getting the disease for the generation when given the carriers ...
...


You propose 25% probability for generations series $-3, -1, 1, ...$. 
You can test this. Select the statistical significance $\alpha = 0.05\%$. Select the appropriate probability density function or teach neural network to it. 

False assumptions


John, his dad, and his wife must all be carriers of the disorder. Why wife should be a carrier?


Literature


Blythe, Peter.  Mathematics for the international student Mathematics HL (Options). Haesa &amp; Harris Publications. I recommend this for beginners. 
TI-84 and TI-86 have good manuals about the statistics. 
Some publications about teaching a neural network doing some of the genetic analysis. 
...