Both Jas and Shane call Marnie their aunt, so they could be cousins.
My explanation: Marnie has an older sibling who is Shanes parent. Marnie had a much younger sister, who was the same age as Shane. They grew up very close, and she married Shanes best friend. So when she had Jas, she made Shane the godfather. When she and her husband died, Shane couldn't cope on his own and decided to move out the the country with Jas.
The photo shows either Jas and her Mom, who looks a lot like Marnie. Or it shows Marnie with her younger sister, who looks a lot like Jas.
I don't think there is any hard evidence, you'll just have to make your own head canon.