As any Mother/Daughter sexual interaction would be incest, a crime, it's not really advertised!
Whenever this topic crops up inevitably the subject of the legality is mentioned. The law is quite interesting in this respect.
The true definition of Incest involves a male having sexual intercourse with a female where there is a blood relationship between the two. Consent is no defence. Incest is still defined by the 1956 Sexual Offences Act and its intention was to prevent procreation within immediate blood lines. This would not apply to the Mother/daughter scenario.
External Link/Members OnlyHowever the Sexual Offences Act 2003 introduced a number of Offences in relation to familial sexual abuse, predominantly in relation to one family member being classified as a child. (sections 24 and 25)
Section 64 is an update to Incest, and covers the situation between two consenting related adults and which now covers penetration involving the vagina or anus by a male using any object or body part, as well as penile penetration of the mouth. Once again not applicable to mother/daughter.
External Link/Members OnlyHowever, section 65 is the interesting one as this is not gender specific
External Link/Members Only65 Sex with an adult relative: consenting to penetration(1) A person aged 16 or over (A) [(subject to subsection (3A))] commits an offence if—
(a) another person (B) penetrates A’s vagina or anus with a part of B’s body or anything else, or penetrates A’s mouth with B’s penis,
(b A consents to the penetration,
(c) the penetration is sexual,
(d) B is aged 18 or over,
(e) A is related to B in a way mentioned in subsection (2), and
(f) A knows or could reasonably be expected to know that he is related to B in that way.
(2) The ways that A may be related to B are as parent, grandparent, child, grandchild, brother, sister, half-brother, half-sister, uncle, aunt, nephew or niece.
The penetration must be sexual (as against medical) and this includes digital penetration. 'Technically' therefore it appears it could be illegal if mother and daughter (sister/sister etc) interact with each other, but only if one penetrates the other in some way? This does not of course mean that prosecution would ever take place?