Accused of sexual abuse

Has anyone successfully coped with either a liar or delusional future mother-in-law? In the future im considering marrying my partner, and adopt her daughter, who i already treat as my own. Her daughter is a funny and loving 4yr old kid who's always ready to speak up her mind. I don't mind strict or protective parents / in-law qualifiers. But these months, my chest has been heavy and heavier, yesterday her grandma did it again. Her grandma keeps insisting that our lil girl said i brought her in the bathroom, had her kiss my bottom, and my front, where white is coming out (yes, the 61 yr old grandma said that in a more ungraceful certain way, many many times over and over to her 4yr old granddaughter). To a point she got the kid to nod a bit in front of the mom, that would normally cause anxiety for my partner, despite my partner knows it's virtually impossible, and i myself would never do such. I used to care strongly for that grandma's well being as well. The sibblings dont seem on planning or taking responbility for her other scandalous behavior, plus i was told they want to protect or preserve her reputation as their mother. But now with all those trauma she's inflicting or passing on the child, i feel like i wanna slap and shout at her face if she acts like that personally. I hate feeling like this, there's so much anger, confusion, sadness, and hopelessness affecting my attention for daily activities like work, reading, or any daily routines, im phasing out more frequently and longer, i think im the one going depressed now. I can't even ask about it with my colleagues, friends, or my family, because it's still my partner's private matter. At least we can handle stuff here as an anonymous group. I try not to overthink, but how can we just let that thought go? It felt unfair, betrayed in away that i cared for the person, but that grandma was like a snake who bit beyond your arms when you were trying to feed em with love. I never tried to look good, but i've doin good for her uncondtionally, i just can't help wonder why a grandma would do such things to her granddaughter. The grandma refuses to speak to me, nor show herself when i invite her to important celebrations, despite her daughter/ my partner dont want her there because of the previous scandalous behaviors. That grandma is the main one who takes care of the kid, while the mom works in office, apparently i'd be having trouble giving personal care for the kid, with all the trust issues, and benefit of the doubt generated. I've been understanding and still trying, since i know the grandma went thru multiple physical and emotional abuse back in her days, not sure if she got raped, but she saw someone got raped in front of her, and personally knows other victims of pedophiles, concubinage, and domestic violence. But it doesnt justify to make lies and harsh concepts for a young child. So do you think i should just keep enduring those accusations and limits they've imposed on me, then let time heal our worries? Or do you have something else in your mind?