I think the key to identifying the birth family (his parents and siblings) of Heinrich/Ernst will be to find a birth/baptism record for him which may name his parents/father.
If you cannot locate one then you could seek indirect records of his birth.
Once you know his parents then you would expect any other births/baptisms to the same parents, in the same location and time period to be his siblings.
You seem to know that he was born March 15, 1867 in "Markendorf, Hannover, Germany" so a starting point to find the records that you seem may be the Hannover Online Genealogy Records of FamilySearch.org:
These are genealogy links to Hannover online databases to assist in
researching your family history. This Online Collection has links to
online databases and indexes that may include birth records, marriage
records, death records, biographies, cemeteries, censuses, histories,
immigration records, land records, military records, newspapers,
obituaries, or probate records.