Since I've developed and RP'ed noble houses in other RP universes, and have done quite a bit of reading into Imperial nobility, going to offer a few thoughts and suggestions for you, Gervas.
You might already know this stuff, especially if you've already read the
Holder article on EVElopedia, but figured I'd offer the advice anyway just in-case.
First thing is: You've chosen a bit of an unconventional Holding, I noticed. Holdings are typically based on land ownership, though land ownership does not mean just planets--it also refers to celestial bodies. You've mentioned mining in your overview, so I'm assuming your bit of space includes asteroid belts and mining colonies, along with any customs stations and other space stations used to administer those, correct? Also, keep in mind that
Holdings are often named after their Holder, so your little area of space would probably be called Heidrich unless it was once owned by a previous Holder House, in which case it may have the former Holder House's name.
Secondly,
keep in mind that Holders, like noble lords in RL feudalism, exist because their lieges wanted them to exist. Every Holder is a vassal to a greater Holder. Before they became a Holder family, they would have had to have proven themselves, and sworn allegiance to, that greater Holder, whom, upon recognizing the merit or worth of the family, deigned to grant them governorship of one of their own Holdings (
whether an already existing one whose Holdership has somehow become empty, or through redefinition of borders in order to create a new Holding). So consider your family's relation to their liege.
Next, be aware that there is only one single titled Holder per noble House. The title of Holder refers specifically to the titled Holder governing a Holding.
Every other member of the House is what is called "non-titled nobility". They are nobles, but they are not referred to as Holders. Therefore, the only person in House Heidrich whom would be a Holder, would be Alexis Heidrich. Gervas would not be a Holder.
In everyday speech, Alexis would be addressed as "Lady Heidrich", while for official announcements at formal proceedings she would likely be styled "The Right Honourable The Holder Heidrich", or simply "The Holder Heidrich" (these styles were used in a recent news report, though sadly due to the reorganization of the EVE Online website, I can't link it atm until they reopen the archive for the individual empire news articles). Gervas would probably be able to be addressed as Lord Heidrich, carrying the Lord preface as a courtesy style (as he is not an actual Holder himself) due to being the husband of a Holder.
Any children the two possess, along with any relatives, would not be referred to as Lord or Lady. Even the heir to the House is not a Holder and would not be referred to as Lord or Lady, unless House Heidrich has a lesser Holding that it can grant to that heir (thus granting a minor Holder title out of courtesy). From how you've described the House, this doesn't seem to be the case, so only Alexis and Gervas would carry the styles Lady and Lord, and only Alexis would carry the title of Holder.
One thing to keep in mind are what I refer to as Custodians. These are members of your Holding, such as the Holder's relatives or favored officials, who have been granted a
Custodial Servitude Contract. A Custodial Servitude Contract grants the contractee direct management over some of the House's slaves. Custodians don't actually own slaves, rather they are granted them as courtesy. Only Alexis would be a slave owner, while Gervas would have to be a Custodian (
unless he goes with ownership of slaves through the SCC by merit of his capsuleer status, which is typically very disrespected by other Holders but is possible if you want him to be able to actually own his own slaves rather than simply being a Custodian over a few of his wife's slaves). Their children would probably be Custodians as well, along with perhaps a few close siblings and/or favored officials.