Genotype phasing with lack of parental information - compound heterozygous

Question

[vntt2407]

Hi, I have a question regarding genotype phasing from exome sequencing results.

So the thing is I've found two heterozygous variants (likely pathogenic) in two patients (siblings) - likely to be compound heterozygous for the disease; however, I can only obtain genotype of the mother (father is deceased)

And my question is how can I confirm/predict with proof about the phasing status of the pathogenic variants (cis or trans)?