Some States in India have laws regulating artificial insemination. The laws pertain to donation by unrelated and often anonymonus donors. They also deal with qualifications of medical personnel. But the question of those laws will come only if the donee suffers something.
Those laws do not deal with donation within family among known persons. If you suspect that she is a "loose character" why do you bother about it. If they are now not willing to give things in writing, leave things as they are. She cannot allege rape after such a long period. She may allege consensual relationship on the basis of false promises. Again the question will come what false promises? Only your brother can bring false charges of adultery against you. As the truth is known to all implied consents of everyone concerned can be assumed.
If they go to police or court against you, you can bring criminal charges against those who go to police or court under IPC-499 and 500. Under these sections they can be punished with imprisonment. They may not be knowing IPC-499 and 500. If you make them know those sections, that may be sufficient to deter them.
There is much debate whether IPC-499 and 500 are reuired in the statute book. At least this is one case where those sections are required. They can make life hell for those accused of.