Liara is likely a granddaughter (or similar) of Dillard's.
In the campaign book Murder in Baldur's Gate there is a listing for Dillard Portyr on page 36 which reads:
The Grand Duke Dillard Portyr
Dillard Portyr is a short, portly man in his sixties. His once black hair is now gray and sparse, and he wears a shabby wig out of habit rather than vanity. A veteran investor, shipping-fleet owner, and trader, Portyr recently pulled back from the business world following a string of sour deals. Now he is using his time to enjoy the comforts that his wealth and title have provided him. Having outlived two wives and three sons, Duke Portyr now lives quietly in a relatively unassuming manor in the Temples district that a handful of devoted servants maintain.
The lore only talks about those relatives which Dillard has outlived. This list is probably not exhaustive (meaning it doesn't mention those relatives who are still living) so it's possible the Liara is a daughter who is still living and simply not mentioned. Liara is described as:
Commanding Fort Beluarian is a lawful evil Flaming Fist blaze (major) named Liara Portyr (see appendix D), who answers directly to Grand Duke Ulder Ravengard of Baldur’s Gate. Liara is 47 years old and has held her post for the past three years.
In the Murder in Baldur's Gate, Ulder Ravengard is a blaze of the Flaming Fist (per the info on page 43). Since he's now a Grand Duke, this means that Tomb's events, canonically, take place after those of Murder at Baldur's Gate.
This Wiki article about Dillard mentions him surviving a coup in 1386. The only source cited on that Wiki page lists the Murder in Baldur's Gate book (pages 4 & 5) but no years are mentioned on those pages (or really anywhere, from what I can see). Yay for bad citations! The information on when this event happened might actually come from one of the novels but I don't have access to these so this is just a guess.
This article about Liara from the same Wiki lists her as being active during the 1480s/1490s. If you peg the year at 1490 (to keep things simple) and do the math, 57 years passed between the coup and Liara's birth. Thus, we know that Dillard was probably in his 70s when she was born.
Given this timeline, Liara likely a granddaughter (or grandniece, etc, if the name is consistent between branches of the family) of Dillard's. Either that, or she's Dillard's daughter and we discover that he was (ahem) performing admirably well past his prime.
If we ignore the poorly cited Wiki articles, we at least know from the sourcebooks cited above that both their lives overlapped that Ulder Ravengard and his transition from blaze to Grand Duke. This confirms that Liara is younger than Dillard and within one or two generations.
Also Lead designer Chris Perkins has confirmed that they are related in a tweet but not indicated the exact nature of their relationship.