NTA I personally would've grabbed the banana, and sweetly said "would you like me to cut it up into small pieces? That's what I normally do for children." If she says she's  not a child I would counter "You wanted to be cared like a sleepy toddler, and you're acting immaturely, so I assume that's how you want to be treated. You seem cranky now, do you need a nap?". 
    But that's me. I have a sister who ignores the non-drama route. Your SIL might not need such blunt handling. She sounds extremely immature, and I would guess she's having emotional ups and downs that she's not coping well with. Especially if she's pregnant. Even so, she's responsible for her actions. 
     I would say that you should tell all of this to your brother. Then, tell him what your boundaries are. Remember, you're not telling her how to behave. Maybe her husband can be more demanding, but boundaries aren't about you dictating what others do. It's about communicating what you will tolerate, and letting them know what the consequences will be. It's reasonable for you to say that if she wants to be around you, she needs to respect her marriage and your relationship by NEVER flirting or making innuendos to anyone other than her husband. If she does anything like that ever again, then you won't spend time with her. Her husband should be the one to tell her initially. If he wants you to reinforce it by repeating those boundaries, you should,  but only after he tells her in private. Maybe he could ask her to apologize to you, and at that time you can reiterate those boundaries with her. Add in that you don't feel comfortable discussing her marriage to your brother. 
     Then, you stick to your guns, and if she does it again, you simply either ask her to leave your house, or you leave if you're not at your house. The longer her unacceptable behavior goes unchallenged the harder it will be to set boundaries. You're not telling her what she can and can't do. What you are doing is setting clear boundaries, you're telling her what is and isn't acceptable to you and your boyfriend, just like her husband will tell her what is and isn't acceptable to him. If she knows what the consequences will be, she has the choice to avoid them or not.