Here’s my translation

Alan

Þau Sveinbjörn ok Steinunn áttu tvá sonu ok fimm dætr. 
They, Sveinbjörn and Steinunn, had two sons and five daughters.

Herdís hét dóttir þeira, er átti Hallr prestr ok lögsögumaðr, sonr Gizurar lögsögumanns. 
Their daughter was-called Herdís, whom Hallr, [the] priest and lawspeaker (nominative), [and] son of Gizurr [the] lawspeaker, married (lit: had [in marriage]).

Helga hét önnur dóttir þeira. 
Their second daughter was-called Helga.

Hana átti Brandr Þórisson ok Helgu Jónsdóttur austan frá Svínafelli. 
Brandr, son of Thórir (nominative) married (lit: had [in marriage]) her and Helga, daughter of Jón, from-the-east beyond (<frá>, Z4) Svinafell (Mountain of Swine). (presumably not both at the same time)

Guðrún hét in þriðja dóttir þeira. 
The third daughter of them was-called Guðrún.

Hana átti Sámr prestr Símonarson. 
Samr [the] priest, son of Símon, (nominative) married (lit: had [in marriage]) her.

Halla hét in fjórða. 
The fourth was-called Halla.

Hana átti Þórðr Arason. 
Þórðr, son of Ari (nominative), married (lit: had [in marriage]) her. .

Birna hét in fimmta. 
The fifth was-called Birna.

Markús hét sonr Sveinbjarnar inn ellri, en Hrafn inn yngri. 
The elder son (all nominative) of Sveinbjörn was called Markús, but (and) the younger (son) Hrafn.

Markús var mikill maðr vexti ok ramr at afli. 
Markús was large man in stature and strong (presumably <ramr>  = <rammr>) in [physical] strength.

Svá var handleggr hans digr meðal axlar ok ölnboga sem lær manns væri. 
His arm was so thick between shoulder and elbow as [if] [it] were [the] thigh of a person (man).

Hann var ungr til bókar settr ok síðan vígðr til prests. 
He was put to school (lit: to books, see also <bók>, Z5) young and after-that ordained (<vígja til e-s>, Z2) a priest.

Á Eyri í Arnarfirði stendr steinn, sá er Markús bar þangat. 
At Eyri in Arnarfjörðr (Arni’s fjord) stands that stone which Markús carried thither.

Í þeim steini váru klappaðir fjórir koppar. 
Into that (<þeim> dative masc sg of <sá>) stone were hewn four cups (nom pl of <koppr>, cup-shaped holes, presumably as washing basins, see next line).

Síðan er hann hafðr til þváttsteins ok er svá höfugr, at varla megu fjórir karlar hefja. 
After-that it (ie the stone, masc) is used (<hafa>, Z4) as a washing stone, and is so heavy that four men can scarcely lift [it].


En fyrir því at skömm eru áföll þessa heims, þá varð Markús eigi langlífr. 
But for that [reason] that [the] heavy-seas (<áfall>, Z1, ie trials and tribulations) of this world (<heimr>, Z2) are short (nom neut pl <skammr>), then Markús became (ie was) not long-lived.

Hann fór einn vetr ór Arnarfirði um heiði til Tálknafjarðar ok þeir fjórir saman. 
He journeyed one winter out-of Anarfjörðr (Arni’s fjord) across [the] heath to Tálknafjörðr (Fjord of whale-bones), and they [were] four together.

Ok á heiðinni gerði at þeim vánt veðr, svá at þeir fóru villt. 
And on the heath [the] weather became bad for them (lit: either (1) [the] weather (nom neut sg) acted (<göra>, Z6) bad for them or (2) an impersonal construction {common when wind or weather is involved}, [it] made (<göra>, Z1, Z12) [the] weather (neut acc sg) bad for them – not sure which but probably the latter), so that they went astray (lit: journeyed wildly, erringly).

Ok er minnstar vánir váru, þá brast undir þeim einn hengiskafl, ok í þessi skriðu týndist Markús prestr ok annarr maðr með honum, en sá komst í brott heill ór skriðunni, er óknástr var. 
And when [their] prospects (ie of finding their way) (<ván>, Z1) were least (ie at their lowest) (or, maybe, when they least expected a snow drift, as per Grace’s translation), then a certain over-hanging-steep-snow-drift broke under them, and in this avalanche (<skriða>) Markús [the] priest perished and another person (man) with him, but that-one (ie he, <sá>) who was weakest (<ú-knár>) made-his-way away hale (ie safe and sound) out-of the-avalanche.

Sá hét Árni, ok Húnþjófr hét annarr maðr, er á brott komst. 
That -one (ie he) was-called Árni, and [the] other person (man) who made-his-way away was-called Húnþjófr.

Þessi atburðr varð þar, sem heitir Feitsdalr.
This event (or accident, <atburðr>, Z3), happened there, where [it] is-called Feitsdalr (Dale of fat?).