The owner has decided to sell the house, which upsets her children alot.
La propietaria ha decidido por vender la casa, lo que/lo cual/ les afecta muchos a sus niños.
The real estate agent, who is very professional, listed the house yesterday.
La agente de bienes raíces, que/quien
/la cual/la que/ es
muy experta, (what's listed?) puso la casa en venta ayer.
I wouldn't use "muy" with "experto". This is already some kind of superlative, so in this case I would rather say "es una experta", or "es muy profesional".
There are many people who use "la/el cual" for a person, but I was taught it is very disrespectful, so if you want to use a more formal sentence than a simple one with "que", use "quien".
The sellling price, which is very reasonable, will help the house to sell quickly.
El precio de salida,
el que/el cual/que/ es muy razonable, ayudará venderla rápidamente la casa.
The kitchen, from which we could smell the spices, was remodeled last year.
La cocina, de la cual/de la que pudimos oler las especias, se remodeló el año pasado.
The living room in which we are seated has a brick fireplace orginal to the house.
La
salón (!) sala [S]en(la)que[/S
] en la que (article is not optional here)/en la cual/ estamos
sentados tiene una chimenea de ladrillo que es orginal
de la casa.
(!) "Salón" es masculino.