My sister is the cheater, is it normal to feel so terrible in keeping her secret?

Three months ago, I came for a surprise visit and saw her making out with her lover instead. It wasn't her husand David of 3 years. I confronted her that same day and she revealed never loving him (he's just her security) and her plans on running away with the lover, whom she's been cheating for 7 months.

I know he doesn't deserve any of this. I'm afraid to admit it but my sister is behaving like a POS and I'm super disappointed in her. The last reunion was awkward. I had to lie about having a bad stomach ache since I wasn't smiling anymore and David noticed something was wrong. I excused myself and left home.

I don't feel like lying about her whearabouts anymore. If I got cheated on or someone was playing with me, I would definitely want to know so I find someone that loves me. She doesn't never him, never did but he does. Even though he's not my blood, it feels terrible to keep such a burden, to have to continue faking smiles and leave in anger. Is it normal to feel so terrible for someone else (not my blood) getting treated like garbage?

My sister is the cheater, is it normal to feel so terrible in keeping her secret?
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