If you haven't already I suggest getting the death certificates for Jacob and Mary from the PA Div.of Vital Records. Check with the Blair County Genealogical Society to see if they have anything and to get their obituaries. The Family History Center in Altoona might also be able to help.
I don't know if you have looked at the census records yet but Jacob and Mary and Margaret are listed in the 1900 census. Oddly it gives Jacob's birth as March 1832, but the month and year are correct for Mary and Margaret. Interestingly it shows Jacob's parents as born in Maryland.
The 1880 census seems more problematic. It does list a Jacob and Mary living in Altoona. He was 45 and she was 35. It also lists 4 daughters. Hopefully the obits will list their children.
The Veterans census of 1890 shows a Jacob living in Altoona. He served in Company G, 11th PA Infantry. Listed next to him a is a William Slaugenhoup, possibly a brother. There's also a Jacob listed in the Civil War pension. His widow was Mary. I suggest getting a copy of the pension as it can contain lots of info on the couple. For that you have to get it from the National Archives in Washington, DC. Also since he obviously was a Civil War veteran his obituary should contain info to confirm we're on the right tract.
The 1860 census shows a Jacob age 29 living with John Slaugenhop age 75, born in Maryland living in Tyrone, Blair County.
Finding them in the other census records seems to be a bit harder even when using soundex. Either the spelling a greatly different or the indexing is bad.
Some of these records you can look at for free at Ancestry. com. For those you can't check with your local libraries to see if they have Ancestry or Heritage Quest.