Both Charles Lamb and his sister Mary suffered from mental illness throughout their lives, although Mary's mental condition was much more severe. Charles greatly loved his sister and felt a deep loyalty to her, even after she killed their mother on September 22, 1796.

The following is a letter that Charles (who was 21 years old at the time) wrote to Samuel Taylor Coleridge on October 5, 1796:

My Dearest Friend,

Your letter was an inestimable treasure to me. It will comfort you I know to know that our prospects are somewhat brighter. My poor dearest Sister, the unhappy and unconscious instrument of the Almighty Judgements to our House, is restored to her senses; to a dreadful sense and recollection of what has passed, awful to her mind, and impressive (as it must be to the end of life) but tempered with religious resignation and the reasonings of a sound judgement which in this early stage knows how to distinguish between a deed committed in a transient fit of frenzy and the terrible guilt of a Mother's murder.

I have seen her. I found her this morning calm and serene, far, very far, from an indecent forgetful serenity. She has a most tender and affectionate concern for what has happened.

God be praised, Coleridge, wonderful as it is to tell, I have never once been otherwise than collected and calm; even on the dreadful day and in the midst of the terrible scene I preserved a tranquility which bystanders may have construed as indifference, a tranquility not of despair; it is folly in me or sin to say it was a religious principle that most supported me.

I allow much to other favourable circumstances. I felt that I had something else to do than to regret. On that first evening my aunt was lying insensible to all appearances like one dying -- my father, with his poor forehead plastered over from a wound he had received from a daughter dearly loved by him who loved him no less dearly -- my Mother a dead and murdered corpse in the next room -- yet I was wonderfully supported.

These mentioned good fortunes and change of prospects had almost brought my mind over to the extremes of the very opposite to despair; I was in danger of making myself too happy; your letter brought me back to a view of things which I had entertained from the beginning; I hope (for Mary I can answer) but I hope that I through life shall never have less recollection nor a fainter impression of what has happened that I have now; 'tis not a light thing, nor meant by the Almighty to be received lightly.

I must be serious, circumspect, and deeply religious through life; by such means may both of us escape madness in future, if it so please the Almighty.

One gets the impression from the subtext of this letter that the Lamb siblings' mother may have had a hand in her own death, perhaps in the form of long-term child abuse. The Lambs' mental ailments may have been genetic, but presumably Mary (who was apparently free of similar murderous "transient fits of frenzy" for the rest of her life) had a reason for murdering her own mother.

Charles' being appointed her legal guardian kept Mary out of the asylum. Neither sibling ever married; Mary's spinsterhood was likely due to her mental illness, but Charles' was possibly partly due to his having a horrible stammer. His stammer apparently limited his professional prospects. He stayed in his fairly low-ranked clerkship at the East India Company for most of his life, and was unable to get a better-respected university job due to his speech impediment.

The siblings did adopt a daughter together whom they named Emma Isola. Emma grew up to marry publisher Edward Moxon.

Coleridge evidently forgave Mary for the murder as well; the two were close friends throughout their lives, and Coleridge praised her intellect and understanding of his ideas.

Charles and Mary lived together until he died in 1834. They collaborated on many writing projects, most notably their 1807 Tales From Shakespeare. Mary, interestingly enough, was the one who handled the Bard's comedies. She died in 1847.

Thanks to sighmoan for the information about Lamb's stammer.