Lady Deech said sexual relationships are privileged 'above all others'
English law favours "idle sexual partners over deserving relatives", Baroness Deech, one of Britain's most senior family lawyers, has said.
She asked why "we treat siblings less favourably than married or civil partners" and why children do not have to care for parents or grandparents.
She noted grandparents should be repaid for the "free" childcare they provide.
Lady Deech's lecture was the latest in a series of speeches on family law at Gresham College in London.
'Unfair on grandparents'
She said: "The extraordinary thing about our tax system is the way in which it privileges sexual relationships of any variety above all others."
She also noted that in terms of inheritance laws, "it is especially odd that a cohabitant who lives with a man and is kept by him has a claim under the Inheritance Act 1975, while a deserving carer daughter does not because she is not, as the act requires, being maintained by him".
Baroness Deech said grandparents face particular unfairness because they are being asked to provide unpaid childcare for working mothers.
She said: "This places particular burdens on grandparents who may need to work themselves, but feel obliged to help out the younger generation.
"They are assuming burdens which deprive them of their own chance to continue to earn a living, and for which they are not compensated, and the childcare they give is no doubt at some cost to them.
"In return for all that grandparents do, should there not be an obligation to keep them, and to keep parents, and reciprocate the care that was given by them to children and grandchildren in their youth?"
The baroness, who is head of the Bar Standards Board, which regulates the work of barristers, described legal precedents that require children to support their parents or grandparents.
Since Elizabeth I, until 1948 in England and Wales, and 1985 in Scotland, sons had to provide for their parents and grandparents throughout their lives, daughters until they married.
In Singapore, "anyone over 60 who is unable to maintain themselves adequately can apply for an order that their child should do so via periodical payments or a lump sum".
However Lady Deech said that "tempting though it seems", the political and cultural landscape of the UK would not make similar legislation possible here.
In March a report by charity Grandparents Plus said grandparents who care for grandchildren should be paid tax credits and given "granny leave" if they work.
The YouGov survey for the charity, which polled more than 2,000 people, found 61% agreed that grandparents should be paid by the government for providing childcare.
Three-quarters said that grandparents of working age who provide childcare should get credit towards their state pension.