Dear Lady Sansa,
rumour has it that you are about to marry Lord Lannister. Are you sure you want to go though with this marriage? But perhaps you don't know all the details about your fiancé. Let me enlighten you so that you don't stumble into a marriage that will only bring you grief.
There is something you should be aware of with regard to your brother's and your mother's death. When your brother had that lethal riding accident Lord Lannister had long been plotting his demise. Thus, it has to be believed that he had his fingers in this affair. You may try to tell yourself that Lord Lannister would not do such a twisted thing, but think of the following: who gained most (and at a low cost) by your brother's death? You should also remember what the Lord Hand has already proven to be capable of in his blood-stained past.
And with regard to your mother – the official version about her death is that, grief-stricken as she was after her son's death, she plunged herself into the churning waters at Riverrun. While it is true that she drowned on the day nobody has seen her commit suicide, and you can assume that someone, some traitor, may well have... lent her a hand, to put it mildly. You could argue that Riverrun was her home, but she was still an important lady of the north, the Lady of Winterfell, so she could still have posed a threat in the Game of Thrones. And you know that Lord Lannister doesn't take any risks.
However, Lord Tywin has got some more negative traits you should take into consideration, Lady Sansa. You know that he has been a widower for many yours, for decades. He did love his first wife madly, and you could only ever expect to live in her shadow. Lord Lannister has never remarried, as you well know – and it has nothing to do with him not finding a theoretically suitable woman in the past. The death of his first wife left him a wreck of a human; what little compassion he may have had died that day alongside with her, and he was never able to be a father again, let alone a loving one. Do you want to have him as an incapable, cold-hearted father for your own children?
And maybe I should not talk about the marriage bed to a refined, sensitive woman like you; yet, you should know the sad truth about Lord Lannister considering him as a... man of certain tastes, for it is one of the worst-kept secrets amongst the more lewd and cunning players in the Game of Thrones. So here we go, and I beg your apologies for telling you about the harsh truth: after his wife's demise he never had another healthy relationship with a woman again. The trauma of being a widower caused him to turn to males, and in the worst possible way – he forced himself on prisoners he had made in the course of his politics... and on corpses after a battle.
I feel deeply ashamed of upsetting a gentle heart like yours, Lady Sansa, but I could not remain quiet in the face of the events looming ahead. I could not bear it if I knew you were degraded by a man like him. Hopefully, you will understand my letter in this light, and I have faith that you will reconsider a union that would surely break your heart and that would condemn you to long-term misery.
Take care, Lady Sansa
A person who only wishes you well