First of all, your friends met your sister through you. In general, there are few opportunities to get to know my sister better. Maybe he will come to see you again and have a few pleasantries with your sister while playing, but that's it. Unless there are other important things, such as what happened to you, he will add your sister's contact information, otherwise he seldom contacts you.
Secondly, excluding other conditions, since he can get your sister's contact information, or he took the initiative to ask for your sister's contact information, so you can see him chatting with your sister, it means that he may really express his love for your sister and want to fall in love with your sister for half a year, which means that the two people should have the same topic and get along well, even if they didn't have this idea at first.
In addition, because there is no specific chat record, we can pay attention to the tone, manner and content of their chat, so that it is easy to judge whether they are in love, whether they can ask directly, indirectly or directly through friends and sisters, or whether they are platitudes, or whether they can judge by their usual expressions, such as whether they feel ambiguous atmosphere and whether they often have physical contact.
In short, because the information you provide is limited, you can't judge completely, but it is possible to speculate on the side, but don't worry. Falling in love is free. If you think your friend is not bad, you can check it out. If not, you can think of something else. No matter what happens, active communication is the best solution.