You mentioned two issues: dating and friendships. Those have similarities and differences.
As for dating - Patterns differ here by individual. Some people date a lot and early and others don't. It's a personality issue involving how you feel about yourself and others. Of course, there's the intimacy issue too. Dating and relationships require a lot of trust, communication and a level of openess often beyond what many people are ready or willing to give. To date, someone else must also find you attractive in some way. That can be physical, mental, emotional &/or spiritual attractiveness. If your sister lacks these, or more importantly, feels she lacks these qualities she is unlikely to be seen as attractive to another person. Heavy and quiet people do date and marry so I don't see these as unsurmountable problems. I also would never assume she is a lesbian just because she hasn't dated. She may lack confidence in her social skills, attractiveness &/or have a very low sex drive.
Friendships - That is more odd to me. If a person has reached the age of 40 and not had a real friend in her life that is abnormal IMO. Since friendship is often the start of a dating relationship this ties in there also. The development of friendships evolves with age. As kids we form bonds over simple matters (favorite toys, same classes, same neighborhood, etc.). With age we begin to select friends based on simple as well as more complex matters also (politics, challenge, intellectual stimulation, etc.). Through this progression of friendships we learn to give and take, learn about ourselves and others and what we want from life. You've heard the saying "no man is an island". Friendships are the basis of a good, happy life. We can be religious, criminals, nice, nasty, different sizes, cultures, etc but to live without friendships is ultimately (and obviously) very lonely.
Do you have friends you could introduce her to or can you include her in activities with others when you go? This could ease her into the process. She may also need some professional help to boost her confidence and social skills. She may not be depressed or express displeasure in her life as it is but she could get so much more out of it if she could connect with someone. It could also help her at work. You say she's a good worker but she may be using that to avoid other social contacts. She may need to learn how to balance work and a social life. Bosses do notice if a person gets along with others. If they don't they could be passed over for promotions or raises even if they are hardworking.