There are a couple of ways splitting up heirlooms can go down. Parents can simply ponder who will get what and write those wishes directly into a will, or they can make the issue a family matter and bring all of their children into the decision. The former dominates in terms of making everything tidy and official, while the latter scores points for making sure everyone is happy with the outcome and less likely to sue the pants off each other later. Your best bet might be a combination of the two.
If you're specifying heirlooms in your will, you'll need to think about three things: who wants what, who deserves what, and how the siblings will react to who did, in fact, get what. For example, while parents may want their firstborn son to inherit their expansive antique dining room set, he might be perfectly happy with the cheap IKEA table and chairs he currently has squeezed into the kitchenette of his apartment. Or perhaps their daughter hopes to inherit her great-grandmother's engagement and wedding rings, but the parents know a daughter-in-law has also admired them for ages. (Here, the general rule of thumb is that if a daughter and daughter-in-law both highly favor an item, the appropriate choice is the daughter. It's all about the bloodline.)
Where it gets dicier is when two siblings can both call valid claim to an item. Perhaps daddy's little girl adored watching her father build the set of family bookshelves, but her brother was a bookworm who was partial to actually reading the books on the shelves. So who gets it?
It's a tough call to make without insight, which is why parents should evaluate their entire stock of heirlooms and consider -- and discuss with their children -- which possessions mean the most to each and who'll be devastated if they miss out on something. Perhaps the daughter did enjoy watching her father build the bookshelves, but she's even more attached to the extensive set of curio pieces he carved when she was a bit older. Boom, problem solved. That's why it can pay to talk it over ahead of time.