I’m expecting my first and my mother is still smoking. She says she only smokes in the backyard but this is near the door. She is very eager for me and the baby to visit at her home and to potentially stay for a few days... I have done some reading about the risks of SUDI and 3rd hand smoke and am concerned this may not be safe. I would like to know if this would be safe for the baby? If not, when could the baby stay and if she were to quit how long after could the baby and I go stay/visit?

Red Nose has the following information about the impact of smoking & infancy.
https://rednose.org.au/article/smoking

Most research into Third Hand Smoke (THS or residual smoke) is fairly recent.
While we are learning more about the effects of THS, there is still work to be done.
We know that Second Hand Smoke (SHS) is a risk factor for Sudden Unexpected Death in Infancy (SUDI) & the impact of second hand smoke on babies is dose related.
A recent meta-analysis of 35 case-control studies reported a dose-response relationship, meaning that the more cigarette smoke the baby is exposed to, the higher the risk of sudden infant death.

Research into THS has primarily been aimed at the residual toxins in furniture, flooring, clothing & hair.
Most papers that I am aware of suggest that if you can smell the residual smoke it may still impact on baby.
Many parents who do smoke outside, will wear a smoking shirt & wash hands &/or hair & change clothes before handling the baby.
Some other responses I have heard is to wait 20mins but I am unsure of the source of that, other reports say if you can smell the residual smoke then the residue is still there.
But is is important to remember the dose relationship - the greater the exposure, the greater the risk.

Some of the concerns in re of THS are:
Children exposed to thirdhand smoke at home are more likely to have:
asthma
ear infections
frequent illnesses
pneumonia

Raising Children Network Australia has the following information
Third-hand smoke is the smoking residue that lands and stays on nearly every surface in the area where someone has been smoking, including on skin, hair, clothing, furniture and flooring.
This means babies and children are still exposed to the harmful toxins from cigarettes, even after adults have finished their cigarettes.
https://raisingchildren.net.au/babies/health-daily-care/health-concerns/second-hand-smoke

This issue is always more difficult when it is related to a family member, as the relationship between grandparents & children is significant in the life of a child.