There are lots of siblings who attend West Point. Lots who attend different Service academies as well.
That said - not all siblings are accepted. It may help a little bit but mostly you need to get in on your own merits.
Admissions should know if you have a sibling. This will show them that you have an idea of what to expect and knowledge about the academy and the West Point experience.
When my younger son applied our area admissions officer made it a point to seek out my older son and ask him point blank if he thought his brother would be able to make it at USMA. He reminds his little brother all the time that he had his fate in his hands for a moment there.... Seriously, it isn't a guarantee, but it certainly carries weight.
I would also share with you that after talking to many parents who have more than 1 West Pointer, it is almost universally agreed that our 2nd had a much harder time plebe year. They thought they knew exactly what to expect because of having observed/listened to their sibling, and discovered that you really can't know until you have lived it. It was almost as if having the preconceived notions made some realities harder. In the end it has brought my sons much closer than they have ever been.