Fixed up some translations here, though I'm not sure about the last one. What's the first word? The text is as follows:

「はいつくばって舌を使って私の足を舐めてきれいにするのよ！」

My J/E dictionary says that the verb 「はいつくばる」 means 'to creep'... hope it helps. =)

Sooo... Sister? Cousin? Childhood friend? Which is it?

anyone knows the source?

Page 46 of Purupuru Monzetsu License.