@Micmac,
It does not matter that they are siblings if you wish to breed them. This, and similar inbreeding of closely related individuals, is how a lot of new morphs are often produced. Bluetongues can live for 20 or more years in captivity and continue to breed throughout their adult lives.
Mating in bluetongues is a fairly brutal affair and can be harsh on either participant. They need to be cooled (to go through hibernation/brumation over the winter months) before breeding to ensure viable eggs and sperm are produced. If kept indoors, the heat provided needs to be reduced during the cooler months. If they have been kept outdoors, this would have happened naturally.
It may not be too late to put them together to mate this year. All you need is a container with a base area of around a ⅓ sq metre. The base should have no substrate and the lizards must be able gain purchase on it with there claws. You can use an appropriated packing box with the bottom flaps folded out and the box placed on carpet or lawn etc. Place the female in first and give it a few minutes to settle down. Then place the male in with her. If he is interested in mating you will know within a matter of minutes. If nothing happens, remove him after 10 mins and try again tomorrow. If he does mate, remove him immediately after he disengages. You can allow them to mate again the following day or two to ensure sperm transfer actually takes place.
It is worth checking out a video on the net of how bluetongues mate so you know what to expect. You need to monitor the process. If feet or tails are grasped, separate them before an injury is done. A cotton bud soaked in mouth wash or metho, and wafted under the nostrils, is very effective in getting them to release their grip if they do not so voluntarily when picked up.
Bear in mind, if you do breed them, you are going to get a lot more than just one youngster.
