Do you know who the biggest problem here is?
Your partner. And then you. The ‘horrible sister’ is irrelevant.
He needs to grow up. You both do.
You walked in to a relationship where you became an instant step mother to a small child who has abandonment and security issues. He’s told you he’d rather live with you than his own parents. This needs dealing with immediately. Your partner should be making his own son feel secure - not you. It all sounds a mess.
He has allowed a member of his family to threaten you with assault and all the other bits and bobs, he’s has allowed this because your just not important enough. This sounds harsh but it’s the reality. The relationship he has with his sister means more to him than the relationship he has with you.
You have enabled him to treat you this way because you have always let it go.
Your in the position now where after you have made a decision on something you have your partner and both sisters manipulating and forcing you it to doing some thing you don’t want to do - this is because you have let your partner place you down at the bottom of the list of priorities.
Unless you are an active Christian and go to church regularly cancel the christening. It means nothing otherwise other than just a party where the kids get dressed up and get given gifts. You can still have people who are active role models to your kids.
You are active in this toxic situation and until you say enough is enough and either put a stop to it or leave you’ll be in this merry go round for a long time.
I really feel sorry for your step son.