Is it normal for a sister to ignore her brother's girlfriend?

I have been with my partner for two years. We are living together and want to spend the rest of our lives together. He is from Germany and his parents do not speak English, nor do I speak German (but I'm learning!). His sister speaks both English and German fluently.

The issue began when I went to Germany to meet his family. After spending a few weeks with his parents (who are really sweet) - I was really looking forward to meeting his sister. She is closer to us in age (in her early 30s) and I was looking forward to being able to speak English with one of his family members.

I had planned ahead and since our visit corresponded with both her birthday and Christmas, I brought small gifts for both as well as a thank for hosting us gift. She didn't say thank you for any of them. Nor did she reciprocate, then or since.

I'm not really sure how to discuss this - in most of these stories there is drama. But in my case, his sister just completely ignores me. I will ask a simple question such as "how are you?" and she will respond to my boyfriend. Sometimes she just stares at me and then after a minute of awkward silence just carries on talking about something else to my boyfriend. She acts as though I never said anything. It seems so weird.

I don't think I've done anything wrong, because we never even had the chance to know each other let alone dislike each other. My boyfriend has noticed as well and doesn't know what to do - he doesn't think talking to her about it will help at all and will only make it worse. He thinks that she might be worried/angry that if we stay together, that he might not move back to Germany. I understand that it's difficult (for us and both of our families), but we are considering moving there - and having the only family member that speaks English act as though I don't exist is a pretty big deterrent. Family is important to me, and I don't want to live in a place where I feel completely isolated and excluded.

It's been almost a year since I met his sister and nothing has changed. On cards and emails she will add in the token "tell her I say hi", but that's it.

Is it normal way for a sister to treat her brother's girlfriend like this?