not making excuses whatsoever but first of all, she's a maid. Second, she is in no way in a position whatsoever to stop FL (forgot her name) from meeting with Alejandro and even socializing with him. Third, what will the society think about a duke bedding a maid and making her his duchess? They would either a.) ridicule the young duke b.) sneer at Ibelina c.) bad rumors will float around them. So basically social suicide.
You might be rebutting the second point with, "well Alejandro's in love with her, she can just say the word and he'll listen". Which I'd say, judging on Ibelina's character, she wants Alejandro to be genuinely happy. And she thinks that being with the FL will make Alejandro happy. And whatever he's feeling with her is just a fickle thing. Aside from what I listed above. With those in mind, who is she to forbid him meeting the FL?
I think people are forgetting that the MC doesn't really think of her self as integrated in the story; she thinks she's an outsider who is reincarnated to make his miserable life a bit better. A proof of that is her being thrust into a position of a maid. Evidently, as what her dialogue says "even if I was reincarnated in this world, and the story shifted a bit, I don't think this [Alejandro liking her] should happen" or something along those lines.
She's really not dumb, but rather, doesn't think she should be meddling too much with the story and just kinda like a deus ex makina to help Alejandro on the burdens he has whatsoever. At this point I really won't be surprised if she think Alejandro as a novel character and wants him to be the main character that will end up with FL in the long run.