Congrats on your new gliders! The babies are old enough to eat grown up food. They are weaned from their mothers around 8 weeks and don't need a milk substitute.
As far as what to feed them, pick a staple diet. Common ones are OHPW (NOT HPW from Exotic Nutrition), BML and TPG. You can Google those or I can give you links or more info if you want. #128513;
Yes, siblings will mate. If you have a Male and a female they will mate, have Joey's and the Joey's will have problems because of inbreeding. You definitely want to get the Male neutered as soon as possible. Depending on the vet, I believe that males can be neutered at 9+ weeks. Some vets have weight restrictions.
If you have any other questions, don't hesitate to ask here, PM me or email me (my email is in my profile).