the veil between the world of the living and the world of the spirits is thinner. They are always close by, but it is easier for them to reach us during the dark half of the year. Traditionally, this is believed to be between Samhain (Halloween) and Beltane (May day).
I seem to sense their presence the strongest from about Fall Equinox through Winter Solstice. To me it feels that the contact becomes less strong from about Imbolc onward (Feb. 2nd or Ground Hog's day) as the growing amount of day light becomes more noticeable.
Any dates given for this in your family's tradition may vary. I think Scotland is farther north than the area of the U.S. that I live in. Latitude definitely affects the amount of day light at a particular location, and historically, pagan/witch belief systems would've evolved in relation to the seasons as experienced at a particular location (if that tradition focused on seasons - I am not sure all of them did).
It is funny, I almost suggested putting the sage oil into a mister bottle with some spring water, and spraying that around. I am glad to be on the same wavelength with that suggestion.
Also, consider that the spirit/entity may have had a reason for coming to you, even if it was your sister's ritual which stirred it up. A woman I used to work for was raised in a tradition which was a mix of voodoo and santeria. She said that when a spirit contacts us that there is nearly always a reason. In order to get the spirit/entity to move on, we need to try and find out what that reason is. Sometimes the spirit/entity has a message for us, or has a request that they need a living person to carry out. As long as the request is within our means, and won't hurt anyone, she said that taking care of the request, or accepting their message for us, is the best way to get them to move on.
Perhaps the message was simply for you to work more at learning or studying the practices or knowledge of your ancestors. I think the old ways want us to wake up and learn and practice them more. This might have been your wake-up call.
Whatever happens, I wish you Bright Blessings!