Jaime and Cersei relationship

[BridgeofFire]

In ASoS Jaime’s chapter, Martin writes 

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Long before his sister’s flowering or the advent of his own manhood, they had seen mares and stallions in the fields and dogs and bitches in the kennels and played at doing the same. 

I was wondering what was the nature of their relationship at this time. By this I mean was it really intimate, because what ever their mother’s maid caught them doing obviously disturbed her.