"My sister isn't Daddy's" Told DH p. 3 MIL said it p.6 Help me talk to DUH p.12

Something really strange happened today and I'm not sure how to take it.

SIL came over today to pick up something. While she was here she was holding YDD and talking to her. Everything was completely normal. I have a good relationship with SIL and we haven't ever really had any issues. I even consider her my friend. She's in the kitchen with me while I'm making dinner and says to me in a whisper
"You know what (insert ODD's name) said to me the other day?"
I asked what she said and she told me ODD said
"(insert YDD's name) isn't Daddy's"
Apparently this happened over the weekend while MIL was watching the kids. I was helping DH with a side job at the time.

You guys she is 4! Where the hell would that have come from? I'm thinking she had to have heard someone say it right? I mean that just doesn't sound like something a 4 year old would say.

For the record, ODD has curly blonde hair and brown eyes and YDD (who is only 7 months) has brown hair and blue eyes. DH has brown hair and brown eyes. I have brown hair and green eyes. Not that any of that really matters.

Sorry if I'm rambling I'm just so thrown off right now. I've never had any issues with MIL before other than her annoying me. This is so random but I just know a 4 year old wouldn't come up with that on her own. Should I talk to ODD about it and try to find out why she said that? Should I bring it up to MIL?