Relationship triangle?

Vyctoria • Mom to 3 girls🎀
If you started dating this guy, he's got a child... but the mother of that child is your sister in laws, sister. (So the baby momma isn't related to you, by anything.) is it right for your sister in law to be pissed? 
We are both happy, my parents are happy, his mom is, our kids are, the only person who isn't, is the sister in law. (Even the baby momma likes me). 
My sister in law was a B at thanksgiving over this, and questioned my sister on things we are doing as a couple and why we are...