Only Isvara can know definitively if the suffering in this life is due to past sins. Humans can't be sure. There are two reasons for this.
Suffering inevitable in this world
Contact of the senses with their objects generates cold and heat,
pleasure and pain. They come and go, being impermanent. Bear with them
patiently, O scion of the Bharata race!
Even a man who has lived his life perfectly will die and endure some suffering.
Isvara distributes the fruits of Karma
The fruit of action is from Him, this being the logical position.
Brahma Sutra Sankaracharya Bhasya III.ii.38
So only Isvara can be certain if a person's suffering is due to past life sins since He is distributing the fruits of Karma.
And isn't helping them a sin too?
Hindu scriptures ask us to help people who are suffering
I abide in all beings as their inner-most soul. Disregarding My
presence within them, men make a show of worshiping Me through images.
If one disregards Me present in all as their soul and Lord but
ignorantly offers worship only to images, such worship is as
ineffective as a sacrificial offering made in ashes. A man who
persecutes Me residing in others, who is proud and haughty, who looks
upon God as the other – such a person will never attain to peace of
mind. If a man disregards and persecutes fellow beings, but worships
Me in images with numerous rituals and rich offerings, I am not at all
pleased with him for proffering such worship. A man should, however,
worship Me in images, side by side with discharging his duties, which
include the love of all beings, until he actually realizes My presence
in in himself and in all beings. As long as man is self-centered and
makes an absolute distinction between himself and others (without
recognising the unity of all in Me, the Inner Pervader), he will be
subject to the great fear of Death (including every form of
deprivation of self-interest). So, overcoming the separateness of a
self-centered life, one should serve all beings with gifts, honor and
love, recognizing that such service is really being rendered to Me who
reside in all beings as their innermost soul.
Srimad Bhagavata Purana III.29.21-27
Need to feel for the afflicted
A Brahmana might be even-sighted and calm in disposition. But if he
cannot sympathize with the afflicted, all the merits of his austerity
come to naught like water kept in a broken pot.
Srimad Bhagavata Purana IV.14.41
A Jiva should not presume to play the role of Isvara and judge another person. He has not been given that right.