Upvote:1
Can she apply? Yes.
Can she enter the UK twice in one year? Possibly. There is no Immigration Rule explicitly stating the number of visits/days permitted. However, applicants do have to demonstrate they are a genuine visitor: V 4.2(b) will not live in the UK for extended periods through frequent or successive visits, or make the UK their main home. https://www.gov.uk/guidance/immigration-rules/immigration-rules-appendix-v-visitor-rules
If your mother has already spent 6 months in the UK in 2019, sheβll likely have a hard time demonstrating that she is a genuine visitor. IMHO sheβd be risking a refusal trying to return so soon after such a long visit. If her visit was less than 6 months she may have a better chance of approval, assuming she can show she has strong ties to home.