Vanakkam,
People deal with grief differently, not everyone will have the same approach. Until your in-laws find closure for their loss, there is not much you can do about it. In this case, time is the best healer.
What you could possibly do is to divert their focus on valuing life, by encouraging them taking up some hobbies, participate in NGOs or temple activities, so that they can have time on their own. A change of environment now and then helps, visiting family members, tours and pilgrimage as one member rightly said.
Psychological support and therapytoo will be very much helpful to help your in-laws getting out of grief. As sometimes, only a neutral third person can be successful on shining light to cases like this.
As the wise member said, do talk to your husband and find out what he thinks on this issue, and whether he has some suggestions that can help your in-laws out of that grief? Do share your thoughts too with him, perhaps you both would agree on an idea or two after the discussion?
As far of shastras, good or bad, do and don'ts, I'd love to hear what other experienced members here have to say. God is benevolent and kind. So He/She does understand the plight of your in-laws and your dilemma. Instead of worrying of the bad effects (if there is), why not pray for speedy recovery of your in-laws and if it makes you feel better say your worries to God and ask God to help you?
I hope your in-laws will be getting better soon.
