冷えるんだったら スカートじゃない ほう が いい ん じゃないのか?
I just don't understand this sentence.
I'm not keen on if ん is の(?) in both cases and I assume ほう（方）simply indicates an action and there's a double negation.
"If it grows cold, a skirt is... (not?) good [to wear]...not?"
Does the character think it should or not be worn for warmth (is it better not to?) (it isn't being worn, for the record) and what about the second ではない(の)? Does it just mean "is that not right?"?