You seem to have a photographic memory to remember what happened when you were 5years old. I don’t blame you. Men are known to have their brains in their erect penises so your brain works overtime every time you have an erection and masturbate.
Do you mean to say when you were 5 years old, your cousin sister was 25 years old. Then how can she be 33 now, if you are 27? Seems your brain is really working hard. At 5 years of age, how do you knpow she was masturbating? She may have had some itching in her pussy and just wanted you to scratch it with your tiny hands.
Ok, now to your question. You say her husband is not coming to her home….then where is he going? See, it is very difficult to relive what happened many years ago, especially for a woman. She has moved on in life and may not even remember the incident you mentioned.
Are you willing to risk her reaction, if you remind her about it. You said you have a plan….but you have not told us what it is. You will have to communicate with her and talk to her, whether she is willing to have sex with you. Till you ask her that, all your plans will be useless.
Why rock the boat? Make friends of your age in your circle. Stop living in a world of Fantasy. You have your life ahead of you. Live in the present not the past.