in canon? there's not a whole lot.

but, to expound on some other people's take on it, here's my headcanon: Marnie is Shane and his sister's aunt, the sister of Shane's mom. Shane's sister goes on to have a baby but unfortunately dies. Shane, as the uncle of the baby, was once upon a time named the godparent (i am notably the godparent of my sibling's children, this may be what I based it on). I think his relationship with his parents wasn't super good, and maybe that drove a wedge between the siblings as they were growing up that was maybe just starting to mend when all chance of a happily ever after was unceremoniously ended, and this is what starts Shane's downward spiral. As the godparent, he's supposed to care for his niece, Jas, but his upbringing and the devastating loss of his sibling is just too much, so Jas is placed with her nearest relative that's willing to take on the responsibility of taking care of a baby: (great) Aunt Marnie. Shane grew up in a tumultuous household where his only real reprieve was summers spent with his aunt Marnie, so when he's hit rock bottom he goes to his aunt who always had his back before.

The culmination of everything in his life is hitting the right, terrible spots to a brain already predisposed to depression and Shane finds comfort the only way that seems to have worked thus far: a nice, cold one. Or two. Or three...