My teacher said that it is strange to say おはようございます to parents, brothers and sisters at home. Is it true?
True for some families and not true for others. Each family is different.
「おはよう」 would be used in more families than 「おはようございます」 is, but this is strictly a personal choice, so calling using the longer version "strange" is a little too much IMHO.
Which one to use can also depend on who is talking to who within the family. Very few, if any, parents would say 「おはようございます」 to their kids, but the reverse is seen more often.
Between siblings, 「おはようございます」 would be very rare, but it is not rare at all to say it to your elder siblings-in-law.
Between older married couples, it is not uncommon for the wives to say 「おはようございます」 to their husbands.
So, in conclusion, what we really never say to family members are 「こんにちは」 and 「こんばんは」 and certainly not 「おはようございます」.