In short, anyone who receives proceeds from the sale or receives a 1099-S will have to report the sale. Typically, the proceeds are divided in accordance with the interests in the property (e.g, 1/6 share each).
You do, however, need to figure the basis for each owner, otherwise, and you have to start with your mother who, after your father passed, presumably succeeded to the own the property in severalty (i.e., she was the sole owner). At that time, the property's basis was stepped up to its fair market value on the date of your father's death. Whether the basis was stepped up for her share only or for in its entirety depends upon whether it was community property.
Subsequently, there was a gift (presumably from your mother) to you and one of your sisters and, later, a gift from you, your mother, and your sister to your other sister. The basis for each needs to be determined in the event they have to report a share of the proceeds from the sale.
You should consult with the closing attorney and/or a local tax professional for this matter.
