Glad I'm not the only one who thinks the AI isn't that bad in RE5. I remember when I first played that game, I was new to that third-person shooter / over-the-shoulder gameplay, and I totally sucked. I wouldn't have made it past the executioner encounter if not for Sheva, who never stopped helping me, even when I was unable to do the same for her. So here's a little Q&A based on my experiences:
Q: Sheva snatches every item from under my nose!
A: Ask for it back. I've never encountered a case where she refused to give me something from her inventory.
Q: Sheva asks for ammo all the time!
A: There are various ways to avoid this problem, for example by having her use different weapons - I recommend rifles, they kill the common enemies in one shot and she rarely misses. Also, she won't stop helping you if you don't give her what she wants, so just ignore it if you're low on ammo. She will quietly fall back on the knife eventually, or the stun rod if she has one (which might be another option worth considering for her, but I haven't tried it yet).
Q: Sheva kills everything and leaves nothing for me / doesn't do anything in a fight!
A: Alternate between her Attack and Cover behaviours. Both have easily noticeable advantages and disadvantages, and both work better in some situations than in others. So just be smart about it.
Q: Sheva never saves me in time when I'm hurt or dying!
A: Try to stay together. Of course she can't reach you from the other end of the world. If that is your main problem, have her on Cover most of the time (which is, if I'm not mistaken, the default setting anyway). Also, try playing RE6 for a while, where your partner pretty much doesn't give a $hit about you dying. Your appreciation of Sheva will increase greatly after that - except if you're playing on Professional. In my experience, it is actually impossible to be saved from dying by the AI on Pro, but I guess the point of the highest difficulty is to make you extra-careful.
Q: Sheva stands in the way when I try to shoot enemies!
A: Turn Attack Reaction off and improve your aim.
Q: I hate Sheva, she's such a stupid character because of her AI!
A: Then prepare to hate Chris too when you play through the game again as Sheva, because he isn't any different. In fact, I think he's even worse, as I have encountered problems with him as the AI that I never had with Sheva. And Chris is a fan favourite, so think before you judge an entire character based on their behaviour as an AI.
In general: Just try to assess every situation and react accordingly. If you're facing the Chainsaw Majini, common sense should tell you that setting Sheva on him with nothing but a stun rod is a bad idea. Also, try to look at her bright side: She has a 100% success rate in QTEs, her gatling gun in the savanna level doesn't seem to overheat, crocodiles completely ignore her, and after unlocking the Who Do You Trust? achievement, she will give you adorable smiles when idle.