Not your problem if she's not getting any. But it will be if you fuck her and your wife finds out.
Either your father-in-law is clueless or he's fucking someone else(either gender). There's a good reason he isn't paying her any attention. You can probably seduce her with minimal risk. Many a man has and didn't get caught,but that doesn't mean you won't. She'll probably be a good fuck too.
I been fucking my mother in law for years since her husband had passed. My wife knew about it and she said she rather have someone she knew fucking her mom instead of some strangers. My wife knew my needs for sex and she can't provide it to me. I go over to her mom and for sex. She also said she rather have me fucking her mom instead of some strangers.
Is great knowing my wife is okay with it and I get all the fucking and sex I need. Yes, older pussy need to be taken care of and I just providing that needs.
My mother in law is not very experienced sexually. She told me her husband basically screwed her for a couple minutes until he got off and that was their sex life. She is still not very good, although we have tried almost everything. At lest she now has orgasms and realizes that sex can be entertaining and fun.