Hello and welcome to the forum. We are so very sorry to hear about your mother in law. How is she doing now? Since your mother in law was at the hospital so quickly, the time factor of taking clotting medication given for stroke doesn't seem to be a factor as to why they didn't give it to her. tPA is only given to those suffering ischemic strokes, however. Was your mother in law hypertensive? Unfortunately though, probably the biggest factor was her age. They sometimes will not give this medication in advanced age. Here is an article that talks about the contraindications in using tPA. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4530420/
Your mother in law's doctor hopefully also gave you an adequate explanation. Please let us know how she is doing.