The biggest hints are probably in the episode where Nadie ignores Ellis much of the time to socialize with Ricardo, and everybody assumes that they're a couple and that Ellis is Ricardo's child from a previous marriage. Ellis spends her time seething in jealousy, and when Nadie tells that they should take a shower before dinner, Ellis simply throws off all her clothes, leading to Nadie's flustered "not together!"
In the same episode, Ellis also asks Nadie if they could be considered a married couple. Ho-hum.
The ending pretty much leaves little to interpret and pretty much cements their relationship. Especially the final scene. When given the option to stay at the little hamlet Ellis and Nadie made a home for themselves at with an elderly married couple. Nadie is quite conflicted about leaving or staying she is beside herself, and it is Ellis who suggests they leave together, when asked why Ellis' exact words are, "Because I love the sparkle in her eye. Love.", Nadie is left taken aback by Ellis' words, but in the next scene they drive off into the sunset together. That to this troper's eyes goes from Romantic Two-Girl Friendship straight to Yuri Genre, or at the very least—gets damn close to it.