What would you do if your wife's sister left a baby behind would you give into foster or take in?

Hi,over the last month there has been pain within family due to the death of my wife's sister she died of cancer leaving a beautiful boy who is a year and a half old.there is no father due to family matters ,the death was sudden and she never told who she wanted her child to stay with.Im with my wife ten years and we have four children we were even trying for another child over the last year until this happened would it be a good idea to do this as in bring up to child together ?Does age matter im 43 my wife's 32? i've not said it to my wife yet because she is torn apart over the death as her and her sister were very close ,Also how do we take claim of the child do we just say we want to be its guardians ?The child is with there grandparents at the moment until court etc of the legal guardians.