In answering a question such as this, we probably should try to avoid being too specific about what the allegorical elements in Rossetti's poem actually stand for. That said, the goblins do appear to represent purveyors of some kind of evil—an ill-gotten benefit, pleasure, or type of wealth, perhaps. In this case, given what you have specified in your question, they would be tantamount to those governments or economic interests that promote colonialism as a means of self-enrichment. The two women, Laura and Lizzie, could conceivably be either the exploited population or the unknowing people of the colonizing country who are being duped into supporting this type of exploitative commerce.

In a colonialist scenario, however, both men and women (especially of the colonized country, of course) are victims. However, if (before considering any other possible meanings) we are to premise our understanding of the poem as one of imperialism as a metaphor for the exploitation of women, Rossetti appears to be predicting the doom of the whole system. One woman ends up resisting the evil ones and thereby saves her sister as well.

An alternative interpretation might see the poem as a more general meditation about good resisting evil. There is a resemblance, given the two female figures, to Coleridge's Christabel, which presents a story similar to that of the Garden of Eden. Rossetti seemingly inverts the scenario of a woman being corrupted into that of a woman being saved through the effort of her sister. Or, if we change the elements of our interpretation slightly, the work is a parable about the generic situation of men (the goblins) victimizing women. In this case, we would be adding a second metaphorical layer, so to speak, by interpreting it also as a depiction of the effects of colonialism specifically upon women.