First of all, I personally would consider that this is a very large family with many connected to RC on FB calling themselves aunts and uncles and cousins.
Secondly, what I have seen posted about uncle/cousin on our threads here at WS appears to be rumor. Someone reported something from another site, not news, not LE.
Thirdly, I must have missed the sister-in-law post; however, that is confusing to me. If he is the son of RC's sister-in -law, he would be nephew to RC, not cousin. Of course, unless he's the son of SC's sister, which would be a nephew by marriage.
Most importantly, I consider that NG is the source saying they have a source close to the family.
In summary, if in fact, LE is looking at a cousin, I suppose there could be a chance that it is this cousin JM, if in fact he is an actual cousin, or it could be many of the other real cousins, or any of the "extended family" who call themselves cousins.
Confusingly, I really have not seen the need to spend much time figuring out the familial relationships. I did see enough evidence to know it is complex.
I am still not sure how it is important except to see if JM is the cousin NG is alluding to in her tease to get more viewers.
JMO