Thomas Hardy and Tryphena Sparks

Hi everybody, Could anyone tell me about the true relationship between Thomas Hardy and his cousin Tryphena Sparks (eleven years his junior)? It is well known that they lived a passionate love story in the year 1867 and some scholars and critics such as Lois Deacon and Terry Coleman in their book Providence and Mr. Hardy went further by declaring that Hardy begot a child on the sixteen year-old cousin. Needless to know that after her death, he dedicated some poems to her, the most revealing and compromising one is: "Thoughts of Phena at News of Her Death". Was it just the imagination of certain scholars and biographers that tries to make up a false event in their lives (Hardy and Tryphena)? or could that be true? Regards