How do I have a relationship with my nieces and nephews when their mom doesn't want us to contact?

Short story short my mom got pregnant at 16 before abortion was legal in the state. My mom got married in her 40s and had me in her 40s so my brother had kids when I was 8-11 years old. The problem is a few years ago my ex sister in law cheated on him left my brother for her coworker, and now hates me because quoting her I’m not the same race and religion as her affair partner was so she doesn’t want contact with me. Has full custody of them. We grew up like siblings and now we are pulled out of each other’s lives without either of our consent.

How do I have a relationship with my nieces and nephews when their mom doesn't want us to contact?
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