This is one of those catch-22's. I think everyone knows somebody who knows somebody of importance. And when it comes to getting into the academy, applicants will do almost anything to gain that edge; and parents will do just about anything to help their child obtain their dream. But at the same time, we all want the satisfaction to know we did it on our own. That when the dust settles, we got in ON OUR OWN! Based on our own accomplishments.
In our small state, it's not uncommon to know a lot of influential people. Including senators and representatives. Matter of fact, when my son was applying to the academy in 2007 we happen to be invited to a friend's home where Ann Romney was coming to visit and help in her husband's 2008 presidential primary run. Our senators were there as well as a lot of other people. Many whom we knew personally. Those who we didn't know personally, we had personal friends introducing us. Our son was with us. Some might even believe that we were "Pimping" our son to all those influential people.
We were so GLAD when my son received an early appointment using his presidential nomination. It was appointed at the end of October of 2007 and he received the official BFE appointment the first week of November. He was also a recruited athlete, HOWEVER, he received the early appointment prior to being recruited for football. This put so many issues to rest. No one could suggest that ANYONE was able to get my son into the academy EXCEPT HIMSELF. He didn't receive MOC nominations. (They offered, but he already had the appointment, so they held the nominations for others in the state). We had individuals writing LOR's that were tied directly to the senators, Representative, and Governor. Again; none of these were needed or used because he already had his appointment. And no one could say that his athletics gave him an unfair edge in receiving an appointment over more "Qualified" applicants, because he became a recruited athlete AFTER receiving his appointment. So for us, knowing how favoritism, politics, and relationships play a role in everyday life; no one can dispute the fact that my son made it into the academy 100% on his own merit, efforts, and hard work. That made him very happy. It also made it very convenient when someone brings up the subject of the best not necessarily getting an appointment because of athletics or favoritism. The pride on my son's face "After the excitement", knowing that HE got into the academy ON HIS OWN EFFORTS, was priceless.
But this brings us back to you. When applying to the academies, this is NOT the time to be HUMBLE and MODEST about your accomplishments. But there is a fine line between proud/accomplished and arrogant. If you believe that you are the best or one of the best in your district and/or state, then you probably won't have any problem finding influential individuals (That your MOC requests) to provide you with letter of recommendations. Most MOC's have a type of LOR that they are looking for. MOST say from someone OTHER than a relative. Some asks for a teacher, classmate, employer, principal, coach, etc... If your MOC specifically asks for an LOR from someone OTHER than a family member, and you have your uncle write one because he personally knows the senator, then that would be in poor taste. However, that's not to say that you don't know someone else that knows you and also the senator who could write you a nice LOR. On the other hand; maybe your MOC's don't stipulate WHO can write the LOR's. In which case, a family member who is a mutual/personal friend of the senator could be allowed. But then you have to decide if it's what you want or not. Other than the senator giving you a nomination or not, no one else will ever know if a particular LOR did it for you or if you did it on your own merit. Again; this is something only you can answer and decide. Also, Remember; EVERY Appointee HAS a NOMINATION; but NOT EVERY NOMINEE receives an APPOINTMENT. Getting the nomination is the first part. And unless your MOC uses the "Principal" nomination method, a nomination doesn't guarantee you an appointment.
I would suggest however that if you do include your uncle's LOR to the senator, that you include a WHOLE LOT MORE LOR's from all the available sources that you MOC allows, and that they be just as meaningful. This way, your uncle's LOR to the senator won't seem so much of a "Consideration" and simply another letter "Validating" the comments of the OTHER LOR's being provided. Best of luck in your decision and your application.