We are trying to put two different things together. What is wrong in having an affiliation to a party. Both are adults, they have the freedom to decide which party they like, whose ideologies and work impresses them. Nothing to prevent or state that members of a family should choose their party affiliations based on what father,mother or a sibling does.
There are such instances wherein family members belong to different parties. People of India and press fondly refers to the Congress party as a dynasty party, even in that if we look, in Mrs.Indira Gandhi's (Congress) family one daughter-in-law and one grandson are in BJP. As long as they stand for their belief and do not indulge in party hopping for the benefits, then it's fine.