Do Spain allow a Long stay Visa for ward?

Where are you moving from?  EU nation, or non-EU?

I'm from the USA and when we applied for our visas, we included my wife's mother in our family.  She is elderly, retired, and has very limited income, and they approved her visa based on us taking financial responsibility for her.

I cannot imagine that they would deny your wife's niece a visa based on that.  She is in the family unit, is essentially like your wife's child, and as such would be considered just like a child. 

If you're applying from a non-EU nation, be advised that you will require documentation that shows the relationship.  Since the girl's birth certificate will not list you or your wife, you'll need to be able to show the consulate or embassy the legal documents that make your wife the guardian of the child.

Additionally, it would be on the safe side to ensure that these documents are translated (by a certified translator) and have apostiles as well.  That way, it is very clear that they are A) legally binding, and B) the state/federal government is vouching for their authenticity.

I am from US, wife and niece are Phlilippine nationals.  We applied; they rejected it at the  Spanish Embassy in Philippines.  They said they don't accept applications for wards. 

We had the guardianship papers, translated and legalized; they didn't look at them.

We'll be bringing her over now under a student visa; work on adoption process (Spain will give visa for adopted child); and move on form there. 

Thanks for your reply and regards