Background: I’ve (21F) been with my husband(21M) for 5 years now (married for 1) and with him came his little sister. We are really close and she considers me as a second mom and I think that’s what makes this situation even harder for me. She turned 17 recently and while she was out with my MIL they ran into an old friend of hers and his son was with him. He is 21 and is in the Navy and was home for Christmas. He said she was pretty and asked my MIL for her number and she gave it to him. They started talking and even after he found out about her age he said he was still interested. After she told me about him I was very against it. I made sure she knew how much I disliked the situation. And she said she stopped talking to him, this was back in December. I came back to my home state for a couple of weeks to get ready for my husbands new duty station and she comes crying to me saying that she loves me and she feels guilty keeping secret from me. I reassured her that there’s no reason to cry and feel guilty that I’ll always be here when she’s ready to talk. She finally told me she never stopped talking to him and they started dating in February. So they’ve been dating for two months now and I do not like it. I told her in exact words “I do not accept your relationship and I will never be ok with it but I am here to listen to you and I will always be here for you no matter if I disagree”. She took my disagreement well but she made me promise not to tell anyone (of course I told my husband anyways).
I guess my question is how do I get over the age gap? How can I be ok with her relationship? Is 17 and 21 even that big of an age gap? Will me telling her that I do not like him ruin our relationship? I’m just worried for her.
TL;DR Is it weird for a 17 yr old and a 21 yr old to be in a romantic relationship?