a song of ice and fire - Who is Lord Hoster Tully's Tansy?

In his death bed, Lord Hoster Tully speaks in delirium of Tansy. Who is she?

There was a smell of death about that room; a heavy smell, sweet and foul, clinging. It reminded her of the sons that she had lost, her sweet Bran and her little Rickon, slain at the hand of Theon Greyjoy, who had been Ned’s ward. She still grieved for Ned, she would always grieve for Ned, but to have her babies taken as well... “It is a monstrous cruel thing to lose a child,” she whispered softly, more to herself than to her father.

Lord Hoster’s eyes opened. “Tansy,” he husked in a voice thick with pain.

He does not know me. Catelyn had grown accustomed to him taking her for her mother or her sister Lysa, but Tansy was a name strange to her. “It’s Catelyn,” she said. “It’s Cat, Father.”

“Forgive me... the blood... oh, please... Tansy...

Lord Hoster groaned. “Dead.” His hand groped for hers. “You’ll have others... sweet babes, and trueborn.”

Answer

I believe what he's talking about is the long rumored notion that he forced Lysa to drink tansy tea, which causes abortions, after he found out she was pregnant with Petyr's child. There's a discussion about it here and here.

As well as a quote from the book:

[Lysa]: “I gave you my maiden’s gift. I would have given you a son too, but they murdered him with moon tea, with tansy and mint and wormwood, a spoon of honey and a drop of permyroyal. It wasn’t me, I never knew, I only drank what Father gave me...”

[Petyr]: “That’s past and done, Lysa. Lord Hoster’s dead, and his old maester as well.”

-A Song of Ice and Fire: A Storm of Swords, Pt. 2 - Blood and Gold (Sansa).