Normally your commissions and profit or loss on investment is two different tax situations. Your commission is typically the same as it would be with any other sale unless your partnership agreement stated otherwise... which often it will.
If you were smart, you would have purchased into an entity and not under personal names / tenants in common. Probably best would be a LLC, possibly an S-Corp and I actually prefer a Land Trust often combined with a LLC. These entities would have a defined ownership percentages of not only cost to acquire but also division of profits or losses at sales, responsibilities at all points of the transaction existence and definition of individual beneficial ownership tax responsibility.
In the Land Trust, usually each individual beneficial owner is responsible for their percentage of the tax consequents. They are given a 1099 at year end and they use that with their own personal tax situation. If the entity has tax consequences it would pay them according to how you have that entity set up and what type it is.
If you do not fully understand these items, you should consult a CPA that is familiar with real estate investment transactions. This WILL NOT include MOST CPA's.... although almost everyone will Tell you They are qualified and knowledgeable. Get references from other investors and you will be far ahead of the game and will have far greater returns in your pocket with far less worry.
After you do this a couple of years, you can try to do your own taxes if you feel comfortable with it, but I still have mine done by a professional and use my knowledge to review it. Far easier to make a few adjustments / clarifications than develop the entire thing.
Besides, if your an investor, you can earn more doing what you do best rather than attempting to work your taxes from scratch.