3 years ago I rescued a pregnant cat, she had her kittens and they nursed on her until they were 3 and half months old, time at which she just 'had enough' and pushed the kittens away.
One of the kittens, Katla, then started to try to nurse on my male cat Elphin. Elphin didn't mind at all and Katla went on with this until she was about 6 months old! Time at which she lost interest in nursing alltogether.
I must say that Elphin and Katla are very bonded and to this day sleep together and groom eachother.
I would not worry about your kitten's behaviour, if both kittens are ok with the situation, it is my experience that it will eventually pass. The only 'side effect' may be that you will have very bonded and inseparable cats.
I personally would not interfeer with what they do, I would just let them be.
Cookie July 1st 1999 f/ RIP July 31st 2008 -cat
Tonali June 3rd 2007 f -cat
Elphin April 21st 2008 m -cat
Stibule(Gino,Airell,Katla,Rasta)June 6th 2009 f- cat
Gino Ferrero April 8th 2010 m -cat
Airell Awenn Awnfenn Ep Berrth April 8th 2010 f- cat
Katla Chilam Balam April 8th 2010 f- cat
Rasta Farian Marley April 9th 2010 m- cat
Chipcat-Filou found June 12th 2011 (Ocicat 6months old) f-cat
Jane the Snow Cat rescued Dec27th 2012 f- cat