Is there any law in the state of Missouri about siblings of the opposite sex(6 year old boy & 3 year old girl) sharing a bedroom?

There are no "laws" there are rules and guidelines used by Children and Family Services if you are being investigated by them. There may be "rules" by apartment complexes or government housing. If you are getting divorced and there is child custody the room arrangement can be bought up in court.

If you tell us what is happening we can discuss your specific issues.

No divorce my sister is apartment hunting & a landlord said he didn't think he could rent to her cause there was a law about opposite sex siblings sharing a room and the only thing I have found is directed towards foster children!

A landlord does not have to and can not be forced. Is this a section 8 ( government assisted housing) rental?

Not section 8! Not wanting 2 force anybody it was just news to me and I couldn't find anything 4 sure so it's always best 2 ask & not assume!

No divorce my sister is apartment hunting & a landlord said he didn't think he could rent 2 her cause there was a law about opposite sex siblings sharing a room and the only thing I have found is directed towards foster children!!

Either the LL doesn't want to rent it and is making up an excuse, or he his honestly mis-informed. In the latter case it would be hard to change his mind, I would guess, absent an attorney's opinion.

a landlord said he didn't think he could rent 2 her cause there was a law about opposite sex siblings sharing a room

So ask the landlord to cite that law.

Such rules are generally on a VERY local basis. Usually no higher than the county or municipality level. They do exist, but are usually hard to find.

Thanks 4 da info!! I'll let my sister know!!

Please keep in mind if the landlord does have others renting from them that are section 8 renters. Then that is where they may have heard about the "law". Im only saying this as many want to jump to a level of discrimination when in fact it is not.

Thanks!!