Ruby's past actions have deeply wounded the family. Her cruel remarks about infertility and the OP's mother's stillbirth cannot be easily forgotten. Attending the baby shower would be a betrayal of the OP's own values and a tacit approval of Ruby's behavior. The family has a right to protect themselves from further emotional harm.
The OP is being unnecessarily vindictive and holding onto grudges. While Ruby's past behavior was inappropriate, she is now pregnant and seeking reconciliation. The OP's boycott of the baby shower is creating further division and punishing the brother and unborn child for Ruby's past mistakes. It is selfish to deny a family member forgiveness and a chance to move forward.
Ruby's behavior toward the OP and her family was undeniably hurtful and unacceptable. Her comments about infertility and the OP's mother's stillbirth were particularly egregious. However, the OP's complete refusal to attend the baby shower and facilitate any form of reconciliation is also contributing to the ongoing conflict. While Ruby should apologize and acknowledge her past wrongdoings, the OP should consider extending an olive branch for the sake of family harmony.