My life isn’t normal...

Idk where to start. My dad cheated on my mom back in 2010 , and had a child from the affair. My dad ended up confessing to my mom about he had done after my younger brother passed away unexpectedly, and wanted forgiveness, but my mom was unwilling to take him back as a romantic partner, and remained friends with him. They remained civil and lived under the same roof (different rooms ofc) because they didn’t want us growing up without having the both of them around at all times. And the culture we come from, would’ve bestowed shame onto our family if what my dad had done, came to light. Till this day, aunts, uncles, cousins, and friends don’t know that I have a half sister.

ONTO MY SITUATION:

My sister is now 9 years old, and lives with us literally 95% of the time. Her mom works and sees her probably twice a month at most...my mom has really taken my half sister in as her own, and does everything for her, cooks, cleans, supports her, takes her out, etc. Bless my mom, for being such an amazing step mom, even though she isn’t romantically involved with my dad (again, they still live together). My dad on the other hand, isn’t doing a good job at parenting my sister. He spoils her and that’s all he knows how to do. Recently, my sister had an attitude with him, and instead of disciplining her and taking away her activities, he just gives her the silent treatment (for 2 days now). It’s such a weird thing to do to a 9 year old. My dad is 54 mentally and physically exhausted and probably doesn’t have the patience to be a parent all over again. My sister has a phone and iPad, and recently, I’ve been noticing that she’s been searching up “gacha lesbian sex stories” and content that isn’t for her age at all!!!. My dad and her biological mom do NOT keep tabs on her iPad and phone, and don’t discipline her because they pitty her for the situation she’s in (being born out of an affair). They just spoil her and I feel like she isn’t being taught right from wrong. I recently went through her messages and she would be texting her friend about my siblings and I, and how we would fight with one another (normal arguing) and she would basically spill our business to people on the internet and I just don’t know what to do right now. Her parents are not being parents and putting her in her place.