To clarify and elaborate a bit.
Before Joan and Maureen became a couple they were sisters-in-law as Maureen was dating Joan's brother (perhaps they were even engaged, possibly married, I don't quite remember as is). Then, some years later, after Joan and Maureen were married, they wanted another child and decided to use Maureen's brother as a sperm donor for Joan to carry their second child (Penny). The reason for this was that their children would be as closely related as possible, biologically, with each other as well as with their mothers.
It might seem a complicated family situation (biologically) but essentially it comes down to this.
Judy and Penny are cousins.
Joan is Judy's aunt and Maureen is Penny's aunt.
Biologically speaking that is.
Just as each Joan's and Maureen's second children will be half-siblings (should Martin be the one to actually impregnate Maureen, Maureen's child would also be Judy's half-sibling and Joan's would be Penny's half-sibling.
Just as there is a chance that Penny's, Megan's and Tiffany's children will all be half-siblings, depending on what the roll of the dice says about Tag being their father or not.