It can be very difficult when children are split up, whether it be through family or Child Protective Services. Many times the oldest sibling wants to either adopt or take full responsibility for the other siblings at the age of 18. While this can be done, it is important to understand the process and your rights.
First, the issue is whether both parents gave up their parental rights or if their parental rights were terminated. If they both gave up their parental rights, then and only then can the children be adopted. However, you must be at least 21 years old to be considered.
Permanent Managing Conservator
If the children haven't been adopted, someone, including your girlfriend, could be appointed as their Permanent Managing Conservator (PMC). This is when a judge appoints a person to be legally responsible for the children without adoption. This is done particularly when both parents have not been terminated or relinquished their rights. As PMC, the person is given rights and responsibilities such as physically possessing the children, providing education, etc. Before becoming PMC a person must go through Child Protective Services to become an approved "foster parent" and after being the foster parent for a minimum of 6 months, then seek court approval to become PMC.
Whether you decide to adopt your siblings or become the PMC, this is a very important decision and should be taken only after serious thought and with a thought out plan of action. The Texas Department of Family and Protective Services has an informative website with additional information.