Functionally, no, Sith can’t have relationships.

They don’t have a rule about it, but their hierarchy is built on dominance, abuse, and backstabbing. Their only interest as an organization is galactic domination, and the only quality they respect is ruthless application of power. Attachments aren’t considered distractions like with the Jedi, but rather vulnerabilities. If you have family you want to protect, then it is in your fellow Siths’ best interest to abduct them, murder them, or turn them against you. If they can keep your child poisoned and near death, but give you a drip-feed treatment that will keep them alive but in agony, they’d do that to ensure your servitude. If they can kill your spouse to eliminate a source of emotional support or induce a sense of despair, they will. You can get married and have kids, but if those kids have Force powers they belong to the state. This is all if you don’t kill your own family.

Different system, same results. The Sith are rewarded vastly more for being career and power-driven, and punished for forming healthy social lives. Moreover, the traits they encourage in their membership aren’t conducive to maintaining healthy connections.