Texas is a community property state much like most of the rest of the southwestern United States. That means that all property acquired after the date of the marriage is community property. Separate property is property that is owned prior to the marriage, property recovered for personal injuries sustained by a spouse during the marriage, and property acquired by gift or inheritance.
In other words, if a wife's relative passes away and she receives a large inheritance then that is her sole separate property and the husband has no claim to those monies if their is a divorce. Typically, the increase in value of separate property is considered separate property. However, there may be a reimbursement due to the other spouse due to contributions made by the other spouse during the marriage to enhance the value of the separate estate.
These community property issues are very complex and a cause of litigation, especially in high asset marital estates.