Hi there, any advice greatly appreciated. My partner has two kids aged 7 and 6. Relations with kids mum isn't great but much better than used to be. She is also in a relationship and has a new baby aged 6 months.

Ok last Sunday stepson apparently put his hand down a little girls pants (not sure if underwear or trousers), little girl told her mum on tuesday who then phoned stepsons mum. Having questioned him he said that he saw daddy doing it to kitty (ie. me)so obviously mum v distressed so called dad on wednesday.
His story was that he sneaked into bedroom at night and to frighten us he climed up under the duvet and in between us and that's when he saw this happen.
We know for a fact that he never witnessed anything like this at our house, we've discussed in detail and no. 1 he has never ever climbed into/onto bed he always taps dad on shoulder to ask if he can get up, no. 2 we'd hear him (creaky door etc), no 3 it would be dark, no. 4 we would feel a six year old in bed..
Anyway my partner has explained to mum the above and that it may be attention with new baby, us getting married soon etc. She has listened and been ok about things but at the end of the day she believes what her son is saying and not his dad which I find a bit distressing as it doesn't really bode well for future issues..also we weren't allowd to take the kids this weekend, we had them for an afternoon instead and she's not sure about this weekend..
Any advice would be greatly appreciated..I don't even know what I need advice for but I feel quite distressed about it.. x