You and our mom other children will inherit the property either through testate or intestate succession.You (and your mom other children)can inherit the property by right of representation in case your mother predeceased.There's no need for her kids to have a SPA because it works by operation of law.
Since you did not state what kind of property does your mother has with her brother/sister, let me presumed that it is a land. Under the Philippine Constitution, as a general rule, alien is prohibited to acquire land. However, this provision accepts some exceptions.One of these is by virtue of succession. Being US citizens (alien) will not matter since you are transferees of said land by virtue of succession.