Advice

Give her ideas, like ask them if they want to play tag or do something fun, she can initiate talking with them.

Round up a group of kids and play a game together

Give her some sweets to share with her little friends then maybe they will ask her to play with them

Sprinkle some sparkling fairy dust glitter on her, little girls loves sparkles. They will swarm your sister like crazy. Tell her to offer some of the sparkles to her friends. It's fairy dust. The key is for her to look shiny more than anyone else. This is gen Z or alpha, you need to do gen Z/alpha shit to get their attention.

Won't that be problematic though?? Some parents may not like their children covered head to toe in glitter because the glitter gets everywhere and the parents might end up getting mad at their child and the child will get mad at the person who gave them glitter??

I would say, if you want to help her, you have to know where the root problem is. For someone who is struggling, even if really young, having someone tell you to "keep trying" or "be positive" are not very helpful and sometimes even detrimental : (ex: you could be putting too much pressure on her without giving her the tools to change her situation. If she "fails" she will think it is her " fault" because she may not be trying hard enough or something)

First, try just listening to her. Have a conversation where you have her open up to you. She's just 5, but her problems are real for her. Ask her indirect questions about the activities she did that day to get contextual info. If she is crying, as her to just tell you what happened and keep asking " WHY?" to follow up questions.
(Ex:"The other kids didn't want to be my friends today" --> WHY --> "They didnt want to sit with me" --> WHY --> " They think/say..." and so on)
*NOTE: Because your sister is young, if she is having issue contextualizing WHY other kids are hesitant to be friends with her, speak to the teacher. They see things with an eyes view and likely have seen many kids in your sister's situation. They can help tell you what is going on*

From there, figure out where the problem is. At a young age, kids in general are very instinctual, and so if your sister is having problems finding friends it could be that the problem is with the way she is interacting with others. Is she too shy? Is she hyperactive and have difficulty with toy sharing or taking turns? Is she interested in different things? Etc.

Once you know what the problem is, it will be easier to help your sister. Don't do the work for her, but as a sister, play an ACTIVE role. Ex: if she is too shy and has a hard time approaching other kids, you can help her practice her social skills by making her break out of her shell in everyday little life situations. Like, if she doesnt do eye contact with new people, make sure she does that when speaking to you or new people she meets in stores and stuff ("You can even turn this into a game: "Tell me what the colour of the eye of the person you are talking to is!")
You can also facilitate socialization by arranging social activities (connecting with families of other kids could help).
Alternatively, if she has a hard time sharing, make sure you practice these skills with her as well.

Anyways... it's hard to help more without actually knowing what the issue is, but I hope this helps. You're a great sister for helping out your sis like this. Just know that it may seem like a little problem right now, but by helping your sis now, you may actually be helping her immensely in the future as well!

Only shitty parents would get mad over some glitters. But then again .. everyone's crazy these days, so yeah... proceed with cautions.
Normal parents wont bother tho, I mean, they are 5 years old, compared to some glitters I think there are more dirtier and stickier things stuck allover them. Like cooties (〜￣△￣)〜

Is this her first experience in a school/group setting (no daycare or preschool)? Going by her age, I'm going to assume she is in kindergarten and has started within the past month. First, telling her "cliché" lines is OKAY, she needs to hear 'Keep trying' and 'It will be alright' so she won't give up. What sounds like a cliché line to an older person, will not sound that way to a 5 year old.
Teach her simple games to play that involve a large group of children--Hide and Seek, Red Light/Green Light, tag, Duck, Duck, Goose, etc. These will be good stepping stones for her, although she will probably need a teacher's help to organize a game with her class.
Modeling positive social behavior is another thing you can do with her--using manners, asking questions/conversing, introductions to new people, inviting people to play, including everyone when playing/talking. This might help ease her anxiety about approaching other children.
All in all, if it is a big concern, your sister's teachers will discuss it with your parents/guardians, or your parents/guardians get into contact with the teachers and ask for advice or if they are observing a problem in school as well.
Children thrust into a new environment might need an adjustment period before they find their place. I'm sure it's hard to see your sister disappointed and lonely, but please continue being supportive! ٩(๑❛ᴗ❛๑)۶

Thank you for the advice everyone