The short answer is that sometimes you can tell from the ISINs that two bonds have the same issuer; but the converse does not work.
Some bonds have a CUSIP assigned (for a small fee when the bond is issued) by the CUSIP authority (which happens to be Standard & Poors). A CUSIP has 9 characters: 6 characters for the issuer; 2 characters for the issue; and 1 checksum digit. If a bond has a CUSIP then its 12-character ISIN is almost certain to contain the country code US, followed by the 9-character cusip, followed by one more checksum digit. So if you see two bonds whose ISINs coincide on characters 3 thru 8, then they're almost certain to have the same issuer.
(However they still might not be pari passou - e.g. one might be senior unsecured, another subordinated... you can't tell the tier from the ISIN).
However the converse does not work for several reasons. One, some issuers issue so many bonds that 2 characters aren't enough for the issue. In this case the CUSIP authority just begins to use another 6-character issuer code. One obvious example is U.S. Treasury. Another example is the Mexican oil company, Petróleos Mexicanos.
Another reason is, sometimes corporate issuers merge, or just change names. In this case, the CUSIP authority assigns new CUSIPs to the equities; but does not change CUSIPs of the existing bonds. However newly issued bonds will use the new 6 issuer code.
Some bonds (e.g. Reg S) might not have any CUSIP assigned. In thic case, their ISIN might start with XS, followed by 10 characters that generally don't have any indication of the issuer.
ISINs of non-USD bonds generally start with a country code, e.g. an ISIN of a Russian rouble denominated bond might have a RU + 10 random-looking characters.
