So sorry to hear your sad news.
What you do about your SILs upcoming birthday depends very much on whether she wants her life to go on as normal, or whether she is feeling that there is no point in anything as she is dying, or whether she needs time to make up her mind whether she believes the prognosis and whether she wants to fight on, or not.
This may not seem a particularly helpful answer, but seen from where I stand, as a wife with a husband dealing with a similar prognosis, I feel that the best love and support we can offered must be based on how the person with the condition and prognosis is feeling right now.
If you and she cannot discuss her illness and the prospects at present, then buy her something nice from the many suggestions given, or invite her out for a short drive in your car, if she is well enough, and you drive, and if you write a card avoid traditional ones that say Happy Birthday or Many Happy Returns, simply put "with love" on a plain card and sign your name.