No. I suggest you look up incest. Its only incest if they've had children together.
That last sentence urgently needs clarification.
Biologically, it's incestuous if and only if the pair doing the snogging are closely related to each other. Legally, it's incestuous if either it's biologically incestuous or they have a legal relationship that's treated as if it were biological. Assuming a man's wife is not a close biological relative, her (full) sister won't be a close biological relative, either. The law on incest, however, treats the wife's sister as if she were the man's sister, too. That's where the terms "sister-in-law", "brother-in-law", etc. Come from. The biological relationship is termed "consanguinity"; the non-biological legal relationship is termed "affinity".