No. The devil is not the cause of every sin because all sins are not committed at the devil's instigation, but some are due to the free-will and the corruption of the flesh.
Diabolus non est causa omnis peccati quia non omnia peccata committuntur Diabolo instigante, sed quaedam ex libertate arbitrii et carnis corruptione.
When man commits sin without being thereto instigated by the devil, he nevertheless becomes a child of the devil thereby, in so far as he imitates him who was the first to sin.
Si qua peccata absque instinctu Diaboli perpetrantur, per ea tamen fiunt homines filii Diaboli, inquantum ipsum primo peccantem imitantur.
Man can of his own accord fall into sin: but he cannot advance in merit without the Divine assistance, which is borne to man by the ministry of the angels. For this reason the angels take part in all our good works: whereas all our sins are not due to the demons' instigation. Nevertheless there is no kind of sin which is not sometimes due to the demons' suggestion.
Homo potest per seipsum ruere in peccatum, sed ad meritum proficere non potest nisi auxilio divino, quod homini exhibetur mediante ministerio Angelorum. Et ideo ad omnia bona nostra cooperantur Angeli, non tamen omnia nostra peccata procedunt ex Daemonum suggestione. Quamvis nullum genus peccati sit, quod non interdum ex Daemonum suggestione proveniat.