A short tandem repeat, also referred to as STR, is DNA code where a letter sequence is repeated. For example, AGTAAGTAAGTA is three repeats of the sequence AGTA. DNA testing that uses STRs are looking for short tandem repeats of DNA that are repeated over and over and those number of repeats are called allele values or markers.
If you share the same number of repeats with another person across a wide range of STR markers then you are related to them and the more you share the more recent your most common ancestor is likely to have been.
A SNP is a single nucleotide polymorphism (pronounced as a single word, “snip”). That means that it is a single small change in your DNA code that rarely mutates. Autosomal DNA tests using SNP can be used to confirm relationships with a high level of accuracy for relationships up to the second cousin level. So it works well for ancestry testing and general population testing but relationships closer than second cousin level requires short tandem repeat DNA testing, also referred to as STR DNA testing.
Companies offering ancestry DNA tests use SNP DNA testing, which is basically general testing of population groups and ethnic profiles, but not specific relationships that are paternal or maternal. If you read the fine print on most ancestry DNA tests it will say, for entertainment purposes only because they cannot provide any kind of specific relationship confirmation for paternity, maternity, sibling, grandparent or avuncular DNA tests.
If you are doing a sibling DNA test to determine if you share one or both parents then the only type of DNA testing that would provide you specific relationship confirmation would be short tandem repeat (STR) DNA testing which is not ancestry DNA testing.
Siblings share 50% of their DNA from their father and 50% of their DNA from their mother. They will have 50% matching short tandem repeats on a chromosome from their mother and 50% matching short tandem repeats from their father. This is a specific relationship test as opposed to a general shared ancestry confirmation.
We all share 99.9% of the same DNA, so its the tiny 0.1% of code sequences that make us unique. These sequences, or genetic markers, are what scientists examine when conducting a sibling DNA test. The more markers tested on a sibling DNA test the higher the accuracy of the results. This is simply mathematical. Bottom line is do not use an ancestry test as a valid sibling DNA test. Use a company that does specific relationship testing using STR DNA testing.