Hashgacha for a in home bakery

Question

Hi Rabbi, My sister in law makes designer cakes and sells them. She is an artist and makes art out of cakes. She makes the cakes in her parents house in their kitchen. She and her parents are modern orthodox. All the ingredients used in the cakes are kosher certified. Does she need a hashgacha, if so is there an inexpensive option (she's not making that much money on the cakes). If she doesnt need one, how should she relate that to customers who keep kosher. Thanks!