Dreams involving family dynamics and the loss of a child can be particularly emotional and complex. Your dream may symbolize various feelings and experiences related to your family, loss, and change.

1. Loss and Grief: The recurring theme of an infant sister dying could reflect unresolved feelings about loss, especially considering the loss of your dad. It might symbolize fear or anxiety about loss in your current life or a processing of grief.
2. Family Dynamics: The presence of your mother remarrying and having children with someone else may indicate feelings about changes in your family structure. It could represent feelings of displacement or changes in your role within the family.
3. Protective Instincts: Watching over the baby and then experiencing its death may evoke feelings of responsibility and fear of not being able to protect those you care about. This can mirror real-life concerns about being able to support or safeguard your loved ones.
4. New Beginnings: The birth of the healthy baby boy at the end of the dream might symbolize hope, renewal, or acceptance of new family dynamics. It suggests that while there may be loss, there can also be new beginnings and opportunities for growth.

Consider reflecting on your feelings about your family situation, past losses, and any changes you are currently experiencing. Journaling or talking with someone about these emotions can also be helpful in processing your dreams and their meanings.