Pls help. I am currently being accused of 'drugging' my partner by my father in law via her drinks /food.

Some background (Not directly related)
My wife has been battling with weight management disorders since her childhood (she is now 40).she also started using high dosages of caffeine pills (supernova thermogenic fat burner pills 550mg).
She also used Alzam (1mg) - sometimes popping 3 daily) as well as various other over the counter pain medication.

Some Pertinent Background:
In January 2016, we both started using methamphetamine recreationally on and off over the past 5 years after she introduced the drug to me. we took the drugs together. We both have previously never touched any illicit drugs.

Like most couples that use drugs, this obviously had a negative impact on our relationship.

My father in law found out about our drug use a few months back after we both came clean during one of our arguments.

We live a few miles away from my father in law and recently whenever my wife is unwell, she would be taken away from my kids and I by her father where she was denied any contact with me whatsoever for any period between 2 weeks and 6 weeks. This practice is still continuing this present moment as she was taken away by her dad a day before lockdown.

She became quite ill around 3 weeks back, her dad immediately took her for a urine drug test which showed up as positive for:
Amphetamines at an alarmingly high concentration level of 17600 ug/ml. The test also indicated an abnormally high concentration of Opiates >8000ng/ml.

I certainly did not spike my wife with any drugs in any way or form and we live with our 2 very young daughters who can be ruled out.
So the 'logic' her father is following is that I am the only remaining logical suspect and he is adamant that my wife has not consumed any drugs of her own accord.

Father in laws conclusion therefore is that since my wife only tests positive for drugs when she is living with me and our kids and not with himq - that I am guilty of drugging my wife. She is perfectly fine when living under his roof. (Although no tests were conducted whilst she lived with him).

He is also adamant that my wife is not an addict as: she 'has no cravings', can account for all money at her disposal, did not steal his car and run off to purchase drugs etc.

Based on his flawed investigation, he considers me a serious threat to his daughters health and safety and as such, he has enough grounds to consider breaking up our 20 year marriage.

I would really appreciate it if anybody with similar experiences or any professional psychologists/therapists etc can offer some advise /guide me on how to prove my innocence in this aeged drugging of my wife.

I'm still in shock at the allegations against me and at this moment my judgement is clouded.
Please accept my apology for my incoherent and lengthy post.

Ps I still love my wife very much and would never do harm to her with intent and want to save my marriage.