Is it morally wrong to date my ex-fiancee's sister?

I was dating/ engaged to a woman for about two years ago. we broke-up about a year ago now, we aren't friends at all anymore. in fact we don't speak anymore.

About 5 months ago, I began hanging out just as friends with my ex's sister, who has a kid. over time we have grown closer, and now I'm wondering if we should take the next step and start dating. I believe she feels the same way as I do, but the only roadblock that there might be is my ex ( who is now 6 months pregnant ) my biggest issue is that my ex would probably freak if anything happened between her sister and i.

So I guess my question is: Do I ignore my feelings and keep the peace in their family, or do I act on my feelings and date my ex's sister, and possibly start world war 3?
Updates:
+1 y
p.s. the child is not mine. I haven't seen my ex since 2 months before she got pregnant
Is it morally wrong to date my ex-fiancee's sister?
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