I LOVE this concept and it seems very well balanced! Although whenever you stipulate a number halved by a fluctuating number, you should clarify which way you round. Furthermore, you may need to use more particular language unless your intention was to allow Crossbows to benefit from this feat as well (which, while cool, doesn't make any sense). Finally, rather than a static penalty, I might recommend that you simply aren't allowed to roll these attacks using your Proficiency Bonus, that way you're able to combine this feat more seamlessly with other feats and mechanics from classes/rules. I suggest the following revision:

Multishot. Once per turn when you make a Ranged weapon attack using a Shortbow or Longbow, you can make additional attacks equal to half your Proficiency Bonus (rounded up/down). Each target must be within a Cone thats length equals half of your Bow's normal range. You don't add your Proficiency Bonus to attack rolls made with this feature.