Working on mystery cousin match who gave me access to her data. She is seeking birth father whose relatives who have tested are our shared matches. Using DNA painter tools and my known relationships, I have narrowed birth father possibilities to my great aunt and uncle's grandsons. Two of their sons married sisters. My mystery cousin has 996 cMs with my 2C. The possibility of mystery cousin and my 2C being 1st cousins is virtually nil. Is it probable that of my 2C's brother, 2 1st cousins, and her double cousin, that the birth father is the double cousin. Would the double relationship passed to mystery cousin result in increased cMs with what should be a 1C1R? My 2C and her son are the only ones who have tested and there is little chance any of her 1st cousins I listed would participate. Sadly, the double cousin died very young from cancer complications and without any children. All of the potential birth fathers likely had opportunity while visiting father and uncles who lived in the area of my mystery cousin's mom.
I’ve made a quick diagram to sort out the relationships that you mention in your question, with descriptors referencing the original poster (OP), 2C = OP’s 2nd cousin, who matches 996 cM with the Mystery Cousin (MC). GA + GU are the OP’s great aunt & uncle, S1 & S2 = their sons, SisA & SisB their wives who are full-sisters, and D the parent of 1C & 1C, 2C’s 1st cousins. DC = double cousin of 2C, and E is her son.
2C and her full-brother 2Cb should share ~866 cM with their 1st cousins. As of yet, no tools similar to DNA Painter are available that incorporate complex relationships like double cousins into their DNA relationship estimates. The DNA Geek is working on one. However, in theory, 2C and 2Cb should share an amount of DNA similar to that of half-siblings ~1759 cM (1160-2436) with their double cousin (DC), because they share two sets of grandparents.
If MC is a child of one of 2C’s 1st cousins, then the relationship between them should be that of 1C1R ~433 (102 – 980) cM. If 2Cb was the father of MC, a full Aunt/niece relationship with 2C would be expected, with cMs matching in the range of 1201 - 2282. If MC is the child of DC, MC’s DNA relationship to 2C would be similar to that of Half Aunt/half niece, which according to DNA Painter is 871 cM (492 – 1315). Their actual DNA match of 996 cM fits well within that range, so DC is the most likely candidate to be MC's biological father.
double 1st cousin have around 2000cm there children have about 1000cm which puts your 900cm at a 1st cousin 1 times removed which clearly lines up with the double 1st cousin line
for reference I have one at about 2100cm and there child at around 1200cm
it just basically doubles all relationships that follow so 1st = half sibling, 1st 1 time = 1st, 2nd = 1st 1 time, and so on