I like the skeletal snake, and I like the idea of tattooing a skeleton shoulder, but yes, I do think that would be visually confusing. If you’re going to tattoo an image of your shoulder’s skeleton, then give the snake flesh and bones. Otherwise, she’s going to get lost in a grayscale of bones.
Is this your first tatt? If so, the other thing I would recommend, not because I’m anti-tattoo (I’m not), but just because it’s hard to visualize how tattoos on joints move until you have it on, I’d suggest finding a good body painter or, even better, mehendi (henna) artist and have him paint it on you. Make it an occasion - go to a party or a club afterwards and see how it feels. See what the reactions are, see how it looks when you catch yourself in a mirror, see what happens to the snake image when you raise your arm over your head - does she look pinched? twisted? constipated?
Then, the next day, go out to breakfast with your grandmother. Can you cover as much of the tatt as you want to? Does your shirt keep slipping to reveal it? Does it show through the material of your shirt? Consider what wardrobe amendments you may need to make if/when you want to work a job where visible tatts are forbidden.
After that, if you still like it, great! Just don’t forget to post pics when you’re done!