Reddit mentions of On Love: A Novel

On Love by Alain de Botton. It's relatively short, but really good. Brings a lot of clarity to the idea of love/being in love.

You might enjoy this delightful little book. It has a section fairly early on (IIRC, read it years ago) where the narrator does the math of him sitting next to such an attractive single woman on an airplane who is also attracted to him.

Alain de Botton's Essays on Love (2006) is an insightful and playful anecdotal narrative that is, according to a review, "profound [in its] observations ... of the pain and exhilaration of love."

It isn't a runaway romance novel, however, though they surely have their place in a desperate share of youthful readership, I feel that they immaturely incite erroneous romantic notions in those inexperienced with love. So if you feel that this factors into how your sister views relationships, this novel is a good check against whirlwind romances, replacing it with a healthy perspective of realism.

And although I don't think that it will appeal via the "tough love" sensibility you're looking for, it does reassuringly articulate the subtleties of relationship dynamics in a very entertaining and palatable way. (It is the only book that I've eager restarted promptly after finishing, should that attest to its readability.) As the novel transitions from discrete stages of the author's relationship from beginning to end, it does feel very much like you're experiencing it, surely in some abstracted form, but one in which the wisdom translates almost unaffected.

In a way, that may be a more forgiving approach to use in guiding her in the right direction, otherwise a "get-your-shit-together" book, as well-written as it may be, would come across as overbearing and consequently diminish the effectiveness of the effort.

Although I am a 20-year old college guy, I feel that my suggestion should merit the credibility equal to the thoughtfulness with which it's written. (Justification. In case my age doesn't reflect my maturity.)

Though perhaps, the similarity in age to your sister would count as a reassuring factor in how the novel would be received by her as well.

Anyways, tell me how it goes.