Teaser

[SouthseaSteel]

I dont like posting these puzzles but I'm hoping fresh RC eyes can help me out of this frustrating jam!!

My brother in law has a new Ancestry Match who shares 52 cM.  Lets call her J.  This was a mildly interesting development.

Next day there was another Match who shares 108 cM.  Lets call her B. Now this is a Top 15 Match so things are getting quite interesting now.

J and B share the same surnames and have sufficiently unique given names that I have built their trees. So I know they were born within 3 years of each other in the 1990s and incidentally, in the same town as my brother in law.  J and B share 1,649 cM so are very probably half siblings rather than a cross generational relationship like aunt/niece or grandmother. I cannot yet connect their tree to our tree.

The main connection we do have is that J shares 313 cM with my brother in law's 2C2R, but her seeming half sister only shares 45 cM with her.  Assuming no pedigree collapse I am struggling to reconcile what relationship two (half) siblings can have with someone sharing 45 and 313 cM. Maybe 2C but thats in the 99.5% decile for 45 cM!!

There are other interesting wrinkles with other Matches but I didnt want to make things too complex straight away.  Any insight into this particular relationship will no doubt assist our endeavours.

Am I missing a scenario?

Thank you

[Glen in Tinsel Kni]

The downside to tree building is the bmd may say one thing but the dna another, and there is always the possibility of being related in more than one way that isn't immediately apparent. 

[Biggles50]

Half Siblings is the most probable.

52cM & 108cM would for me suggest a 1/2 2C1R if they are indeed a generation removed from your BiL.

You cannot assume No Pedigree Collapse, the cM values point towards it being a probability.

The two matches may not even know that a parent or parents is not biologically related to them.

Trust DNA.

Have you tried Messaging?

[SouthseaSteel]

Thanks both!!  Dont worry I trust DNA, the good thing about it is that it is right whether or not you believe it!!

I have contacted one Match, who had no tree, and she hasnt responded.  I was going to contact the second, who has a large maternal linked tree but nothing paternally, but was awaiting any suggestions from this posting to help me target my message.

Particularly so, for as you suggest I notice both Matches have the same family name and the same mother's maiden name but are in fact half sisters by DNA.  They may already have known this but then again maybe they didnt .....

Tx 

[4b2]

If you know their tree, I imagine from BMD index reverse-engineering, are they apparent full siblings or not?

I've learned to pay little attention to what I think the cM is most likely and go with the data from DNA Painter. Sometimes you just get matches right at the end of the spectrum. 45 and 313cM can still be 2nd cousins.

[SouthseaSteel]

Tx.  Their trees suggest they are full siblings but they are not as DNA says they arent!! Two sisters taking a test at the same time does beg questions!!

As my BiL is related to both by DNA I presume that he is related to the parent they have in common but unfortunately I dont know which one that is.  I have now contacted both in a very generic manner but nada from either response so far.

I will try WATO but apart from this 313/45 cM shared Match I mention in the OP the others are in the grass at 30 odd cM and less