She's sandwiched between a perfect teenager ("sweet and loving," "treats her brother so nicely and has a lot of patience with him" etc.) and a kid barely out of toddlerhood who "needs more attention" because of his age.
So she's left to do whatever she can to get attention from you, from dad, from older half-sister and from younger half-brother. While it's not an excuse, it IS an explanation, and it's an explanation you seem to be overlooking. Yes, you say that you try to give each kid individual time alone with you so they get attention and while that's positive and smart of you, it's not always going to be adequate for this girl, at her age.
Please check into some parenting books that describe "ages and stages." On here, a lot of us recommend books for understanding younger kids' stages but we don't often think about what kids your stepdaughter's age are going through.
In her case: Try to think like the 11-year-old a bit to gain some perspective and compassion. To her, the cool, sweet older sister is both someone she desperately wants to impress somehow (hence the cursing, because impressing Big Sister negatively is still impressing her, right?) and at the same time, someone who gets to swoop in twice a year and be a perfect delight to the adults when younger sister knows she, herself, isn't always delightful. Have you ever said to the younger girl, "Why can't you be more like Older Sister?" If not in those words -- have you or dad implied it, or held older sister up as a paragon of "sweet and loving and patient with her brother"? Please take care that you don't. That might work with a young child but not with a girl of 11 - it will only set her against the idea of being anything at all like her sister.
Also, she is out for attention versus her younger brother, who is still at a cute age. I wouldn't ask a kid her age to give a child that young "respect" -- I'd put it more as "Stay out of his hair and he will stay out of yours." Separation, not togetherness, is sometimes best for a long while when the kids have that big an age difference and the older one knows the younger one is going to get the lion's share of attention. Again, try to see him from her perspective. You say she's a mother hen (and seem to mean it negatively, so I take it you mean it as "bossy") but be sure -- Do you ever expect her to look after him? Ask her or tell her to watch him while you do an errand or chore? Again, take care that you're not telling her she's expected to care for him as part of her sisterly duties, then nagging her when she goes overboard. I truly would be sure that they do not have too much togetherness, period -- both because it will reduce the bickering and because the 11 year old girl deserves to have some things and time of her own, without little brother in tow (or older sister getting all the attention "because she's only here a few weeks each year" etc.).
Does the girl have activities that are hers and hers alone? A hobby or two? Does she have an outside, non-electronic, non-school activity such as scouting, a drama club, a dance class, marital art, a sport, art, whatever? If she does not-- please consult with her and let her choose something to do, and support her in it loud and long and without letting things get in the way. By that I mean: She shouldn't hear "Brother has kiddie soccer so you have to shift to another dance class" or "Sister's in town this summer so you can't attend the softball team picnic" and so on. Maybe she already has activities; if so, sorry, I'm just wondering if she has something all her own that she's good at and enjoys. If she does, or if you help her find that something and support it strongly, it could help.
Finally, please, please be aware - I'm speaking as the mom of a girl of 13 here -- what she is doing is totally, utterly typical for any girl of 11, even the nicest ones. They all go through times of seeming "somewhat narcissistic." They are still the centers of their own universes (as is any younger child) but they are also stressed by the slow realization that they are not the center of everyone else's universes, and they don't quite know how to process that. So they boss younger siblings, and try to impress older ones, and generally flail for mom and dad's attention -- and yes, even if she is using negative behaviors to get your attention -- she is still after your attention.
Point out every little thing she does right. Even things that you, as an adult, feel should be "stuff we just do every day that doesn't deserve a special thank you!" Give her more attention than you think she needs. Be sure to encourage her to have her own friends her own age over to do things, and offer to take her and a friend someplace, without brother or sister along. It sounds like coddling, and in a sense it is -- but she is feeling lost in the middle here.
And when she does things that are clearly defined as beyond the pale, ensure that she knows in advance what the consequence will be and then make that consequence happen without fuss or "getting on her case." Swift and clear and known in advance by everyone including her --that's how the consequences need to be. But spend much more energy and time finding things she does right and praising them.
