If Jordan has blood type O and his sister has type AB, what would the blood type

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The parents would be type A and type B. We can even tell their types genetically--AO and BO.

Everyone has two copies of the ABO bloodtype gene. A and B are dominant over O, so someone with any A copies shows up with type A in their blood, and someone with any B copies shows up with type B in their blood. This means that AO and AA both show up as A; the "O" is masked by the presence of the A.

Because one child is AB, we know that they have to have AB genetically--that's the only way for both types to show up at once. The other child is OO, because the presence of any other gene (A or B) would mask the O, so the only way to have O type blood is to have OO type genes.

Each child gets one copy of the gene from each parent. Because one child is OO, we know that each parent has at least one copy of O. The other child is AB, so we know that at least one parent carries one A copy, and the other parent carries at least one B copy. This makes the parents AO and BO genetically, which give blood types A and blood types B.

Don't attack someone for mistaking where to post something--they obviously just ended up in the wrong section. I serious doubt that the person asking the question was trying to insult genealogy in some obscure way.

That said, I disagree with the big genealogy versus genetics argument. Blood type is a simple method that could be used to check on paternity in records when the historical records are unclear. It isn't a 100% proof, but it could be used to eliminate parents/children/etc. when the history records aren't completely clear (the records provide a limited possible group of parents, and this--with other easy to track inherited traits--could narrow the group down). Sure, it's based on genetics, but why ignore a useful tool when it helps out?

This sort of genetics isn't so complicated that it requires a doctorate to understand the process. It also doesn't require a lab to check on these inherited traits. Most genealogical records today won't carry blood type information, but some may, and there are other clearly defined traits that could be used from photographs or paintings. For example, cleft chin inheritance is clearly defined genetically, and would be visible in early photographs and possibly even earlier paintings (assuming the painting could be considered accurate).

In both positive and negative; the parents could be A, B, AB, and O

This is the genealogy forum. That is NOT a genealogy question. It is genetics. They are NOT the same thing, and are not related.

Genealogy is a HISTORICAL body of research of one's deceased ancestors, and is about birth/marriage/death dates and places, what countries our ancestors came from, how our ancestors affected and impacted history, etc; and uses HISTORICAL documents such as birth certificates, marriage certificates, death certificates, tax records, land records, court records, church records, wills and probate records, etc., to back up and document our research, and anyone can do it if they work at it.

Genetics is a SCIENCE that requires years of college and specialized training to do, and is concerned with genes, microbiology, etc., and uses SCIENTIFIC methods and experiments to prove or disprove theory or to show cause and effect. Research is done in a lab, not in the records basement of court houses and churches.

Genealogists are not geneticists.

I agree with the genetics vs genealogy, but I shall try to answer your question anyway.

Their parents would be AO and BO. So essentially they would be types A and B carrying the O.