I saw my dad intimately kissing my 16 year old sister

A few days ago, our maid had a day off because it was the weekend. It was just me, my sister, and our dad at home. My mom was out running errands at the time. I went downstairs to get something when I caught them kissing on the lips. It wasn’t an innocent peck. They were kissing intimately, and I froze when I saw them. I couldn’t believe what I was witnessing. Later, I told my mom about it, hoping she’d take me seriously. But she just said I was probably seeing things and had misunderstood what I saw. I tried explaining, but she brushed me off and changed the subject. Now I’m questioning everything. I don’t understand why my mom would rather dismiss me than even investigate. I feel sick keeping this to myself, but I’m scared nobody else will believe me either.