If a sparse language is NP complete, then are all languages in NP in P/poly?
Yes, see Mahaney's theorem. It states that if a sparse language is $\mathbf{NP}-$complete, then $\mathbf{P=NP}$, and since $\mathbf{P\subseteq P_{poly}}$, then $\mathbf{NP\subseteq P_{poly}}$