Should someone who suffers from obsessive thoughts about the mixing of lineages as a result of using the toilet seat after her brother, while being pregnant, turn away from these thoughts and ignore them?

There is no justification for this fear. You must turn away from these whisperings (waswas), be convinced by this answer, and be wary of letting them persist, as their consequences are severe. What will help you with this is seeking Allah’s help, sincerely turning to Him, and supplicating frequently.