Thank you for using justanswer. You've actually asked a few different questions here, so let's take them one a time.
Since IRS really has no way to know who is actually entitled to claim the children on their tax retun, they will honor the claim they receive first. However, this does NOT stop you from also filing for the children. (you will have to mail the tax return instead of elelctronically filing it however) If you have already filed your return, you may amend it by filing Form 1040X .
Unless there is something in your divorce decree that states your ex may claim the children on alternate years, then the custodial parent is the person enetitled to claim the children and all of the tax credits associated with them, such as the Child Tax Credit or the Earned Income Credit if applicable.
Once you send in a return claiming the children, the IRS will notify both you and your ex that more than 1 person claimed the same children, and would 1 of your like to amend and remove the children as dependents. If neither of you does, then the IRS will send out a packet of information designed to gather information to see who is really entitled to claim the children. The packet will contain such things as whose name is XXXXX XXXXX school records as being the custodial parent, what neighborhood do the children attend school in, who gets notified if there's an accident, who goes to parent/teacher conferences, who takes them to the doctor,, etc. If you are receiving any aid from the state, such as food stamps or medicaid for exemple, that would be an excellent proof of where the child's main home is.
If it is determined that you were in fact entitled to claim the children, then your claim will be honored, and your ex will have to pay back any money he was paid for associated with claiming the children. Please see below for more in depth information:
As for the land you purchased from your sister, and are now selling back to her, the capital gains tax law no longer allow you to "defer" or postpone paying the tax just because you reinvest it. I'm assuming you owned the land for more than a year, which makes the profit on the sale eligible for the lower capital gains tax rate (15%).
Tax Facts About Capital Gains and Losses
Finally when it comes to your family home, since in effect your ex is buying you out, there should be no tax consequences as long as your owned and lived in the home at least 2 out of the last 5 years. Again, please see below:
Publication 523 (2008), Selling Your Home
Excluding the Gain
I hope this helps.