Difficult sentences from the book Washington Square

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I'm reading Washington Square but having trouble understanding the bold parts of following sentences:

Mrs Penniman's own account of the matter was that she had remained to take charge of her niece's education. she had given this account, at least, to everyone but the Doctor, who never asked for explanations which he could entertain himself any day with inventing.

she had not a high sense of humor, but she had enough to prevent her from making this mistake; and her borther, on his side, had enough to excuse her, in her situation, for laying him under contribution during a considerable part of lifetime.

His assent could only be tacit, for he had never been dazzled by his isster's intellectual lustre.

He contented himself with making known, very distinctly in the form of a lucid ultimatum, his wishes with regard to Catherine.

She possibly reflected that her own great use in the world was owing to her aptitude for many things.[/i]