Can a widow claim share in the property of deceased husband

one of my sister is widow and has two children .they all have their share in in-laws property as it was her husband's..they are only 4 members left she with kids and her mother in-law. her in-laws family or relatives are not willing to give her full share and playing games.. actually there is one plot of her husband's which is joint with someone..now after her husband all 4 have their share in their part of plot.. before dividing her in law's want to sell it with joint member they asked my sister if she want her share she has to take legal custody of kids and for that has to sign agreement in which she has to agree to made an fd for both kids on her own first only then they can sell plot and give her her share..they want money to clear their own debt.. they want her to give that money ...she does not has any strong way of income.. what she can do now.... also want to know that if she will has legal custody what rights she can have about property of her kids..