AITA for not fully supporting my sister after my dad abandoned her?

A 16-year-old girl is struggling to support her 15-year-old half-sister after a DNA test revealed their mother had an affair, meaning their father is not the biological father of the younger sister. The father moved out immediately, is pursuing divorce, and has cut off all emotional contact with the younger daughter while still fighting for partial custody of the older girl. The sisters, who were once extremely close, are now both hurting deeply—the younger one feels completely abandoned, while the older one is furious at her mother and conflicted about her father’s actions.

What makes the situation more complicated is that the younger sister recently tried to secretly take and destroy a box of their paternal grandmother’s expensive heirloom jewelry as an act of anger and revenge. When the older sister intervened and explained why the jewelry should be preserved, the younger sister accused her of siding with their father and questioned whether they should even consider themselves sisters anymore.

‘AITA for not fully supporting my sister after my dad abandoned her?’

The discovery began with a DNA test the sisters bought themselves.

The results showed the younger sister was not biologically related to their Polish father.

The father now completely ignores the younger sister while pursuing custody of the older one.

The conflict escalated when the younger sister tried to destroy cherished family heirlooms.

The younger sister’s pain is completely understandable—she has lost the only father she has ever known and feels rejected for something she had no control over. Her impulse to destroy the jewelry is an angry, symbolic attack on the family identity that no longer includes her. While the act itself is wrong (theft and destruction of property), it stems from overwhelming grief and rage.

The older sister is also hurting, torn between loyalty to the father who raised her and love for the sister she grew up with as a twin-like best friend. Trying to protect meaningful heirlooms is reasonable, but the timing and delivery of that message likely felt like choosing sides. Family therapists stress that teenagers in this situation desperately need professional support—individual therapy for each child to process grief, anger, and identity issues, and family therapy (initially without the mother) to rebuild sibling and father-child bonds.

The father’s reaction, while human and painful, risks causing long-term emotional harm by punishing a child for the mother’s actions. The older sister is not wrong to maintain her relationship with her father, but she can also clearly communicate to her sister that her love and sisterhood remain unchanged regardless of DNA. The real person responsible for this pain is the mother, whose choices created the crisis.

See what others had to share with OP:

Most users agreed the older sister is not the asshole for stopping the destruction of the jewelry, but emphasized that everyone needs therapy.

Several comments placed the primary blame on the mother while acknowledging the father’s pain.

A few opinions focused on the practical and legal risks of the sister’s actions.

The older sister is not wrong for protecting the grandmother’s jewelry—those heirlooms are meaningful family history, not just the father’s property. At the same time, the younger sister’s pain is valid and intense; her actions come from deep hurt rather than malice. The real source of this family fracture is the mother’s long-hidden affair and the father’s decision to emotionally abandon a child he raised for 15 years. Therapy (individual and family) is urgently needed to help everyone process the betrayal, grief, and shifting identities without further damaging sibling bonds.

How would you try to reassure a younger sibling in this situation? Do you think the father should be expected to continue treating the younger daughter as his own, or is his withdrawal understandable even if painful? Should the older sister limit time with her father to avoid hurting her sister further? Share your thoughts in the comments below.