How to account for capital gains for a JTWROS Brokerage account with separate tax files?

[ogilvytn]

I have a JTWROS brokerage account with a family member. 
Last year we sold assets on the account and it generated a 1099-B.

I have losses I can use the capital gain against to yield a lower tax than if my family member did on their taxes. 

They are currently 'primary' while I'm second in the ordering of names in the account. Both of our SSNs were submitted during account creation. The generated 1099-B PDF has their SSN listed in the upper right corner.

If it matters, the bulk of the assets were bought by me (on a different individual account, multiple years prior)

Can I file 100% of the gains income against my return? Or does it have to be the 'primary' one's tax file? What's required for me to file it instead of them?

[Critter-3]

Unless you put the account into a partnership where you can divide the income as you like between the 2 of you (and is the correct way to handle this when you have non spouses on a joint account)   then the path of least resistance is for the primary account holder whose SS# is on the tax forms should report the entire amount on their return.   If they complete their  return without that info  and then add that broker's info  and see the difference it makes to the bottom line of the return with and without that amount ...  then they can  figure out the % of that difference that belongs to you so you can "settle up" with the other person.  

[rjs]

The method that Critter-3 suggested will not let you use your capital losses to offset the gain from the sale in the joint account. Here's another way to do it, which might actually be simpler than calculating the tax effect of the sale and "settling up" informally.

The family member does have to report the full amount of the sale on their tax return. But they received the 1099-B as a nominee for you. They can report the sale with a nominee adjustment (on Form 8949) for your share of the gain. The adjustment will remove your share of the gain from their tax return. You then report your share of the sale on your tax return as an investment sale for which you did not receive a 1099-B. That will put your share of the capital gain on your tax return, where it can be partially offset by your capital losses.

If it happens that your tax rate on capital gains is not the same as your family member's rate, the nominee procedure will also insure that your share of the gain is taxed at your rate.