Your mom sucks and moving away may be the best thing.
Dear Prudence,
My fiancée and I grew up in the same small city. We went to different high schools but attended the same college. My fiancée doesn’t have any living grandparents. My mother is very nosy and did some digging in my fiancée’s family. It turns out that her maternal grandmother is the sister of my paternal grandmother, making us second cousins. Everyone was mildly shocked by the connection, but my fiancée and I really don’t care. My mother is freaking out about the “incest” and has made “flipper baby” comments. My sisters have made “keeping it in the family” jokes, and it is very irritating. No matter how much I tell them to quit it, they won’t. My fiancée thinks they don’t like her and all this is a smoke screen. We have plans to move out of state so she can pursue her master’s. Is she right? How do I deal with my family?
—Not a Punchline
Re: Your mom sucks and moving away may be the best thing.
I'd draw a line. "Please stop or I will be limiting communication with you moving forward. I don't think this is funny (to sister) or a matter of your concern (to mother)."
IF it's legal to do this (and I've never looked into it) then you can tell your mom to stop however if a genetic counselor has some feedback about this then please consider it.
That said, whether or not it's genetically a non-issue is really case dependent on each person.
I'll cite the issue of BIL and SIL who last year found out that they're both recessive carriers of a gene and it results in a 25% chance of any child they conceive to have birth defects that mean that their child has an extremely high risk of miscarriage, stillbirth and will not live for more than days after birth if that. Their daughter last year lived 32 minutes. More and more genetic testing is finding that there can be concerns not for the 1st generation and not the 2nd but even the 3rd. That's the point I am making here.
Like @STARMOON44 mentioned, genetically it is a non issue. If the LW hasn't already, they should find a credible source/article that explains that and send it to their family. JIC it's a concern the mother or sister have.
Not that I've usually had that tactic work for me when someone is mistaken about "black and white" facts, lol. But at least I've done my due diligence to show I'm giving correct information.
Tell your sisters to knock off the jokes.
But it wouldn't hurt, and could even be helpful, if you get genetic counseling to find out the odds that your children could inherit two recessive genes for a genetic disorder.