When you say "union", do you mean it's just a slip together joint? A union is a plumbing fixture that is a screw together connector that can be unscrewed to disassemble the joint. If it is a slip together joint, then is should be soldered. Brass and copper are the same basic metals and are joined with a solder joint. Problem is, the cast brass will be at least 3" and possibly 4" and is quite heavy. This will not be a solder job for a novice. You really will need a plumber to make this connection. There is no sealant that will suffice, including the liquid solder that is hyped to replace traditional heat. And no, silicone won't work either. You have gotten by in the past with this slip joint because there is no pressure in a drain line.