My never-married, prickly, eccentric sister has developed a history of being what I call an emotional vulture, to put it bluntly. She has always been eccentric and has had difficulty with maintaining a relationship with a man of her own and has not been in an actual relationship herself for more than 25 years. She is now in her 60's.

She has developed a habit of zoning in on other people's relationships and interfering. She usually starts it out as being "helpful" and offering assistance. She has had two female friends who have had serious illnesses (both of whom eventually died) where my sister has "helped" out the husbands and/or adult kids. She has even done this to our mother when she was alive - got involved with my mom's elderly boyfriend after my mother had a suffered a debilitating illness.

She claims there is nothing going with any of these men and families other than being "helpful" to to them all. Not to be spiteful, but that may or may not be true as she is not an attractive person either physically or personality-wise. She is very opinionated, bossy and nasty when she doesn't get her way or others don't take her advice as she is trying to "help" them. She is very good at conning people with her offers of "helpfullness" only to use that as an entry way to gather information about the people, insert herself into their families/relationships and begin her manipulative behavior.

My boyfriend is well aware of this background and shares my concerns about her behavior and thinks it's very strange and problematic. Unfortunately, my sister has now zoned in on my boyfriend's family. She has only met them all on one occasion when they were here visiting as they live a long distance away. After that one meeting, she then took the standard, "Come and visit us sometime" literally and actually did invite herself to a visit with them all as she was going on a trip in that general area - but it did require her to buy an extra plane ticket to make that leg of the journey. So it wasn't like she was driving by their town and stopped by. Neither my boyfriend or I were there at the time of her visit.

Of course, my boyfriend's family were welcoming and gracious (they are of another culture) and hosted her in their home for a few days. After that visit, she is now e-mailing them regularly, sending letters, photos, gifts for no reason, etc. Since they don't know her history and are from another culture, I think that they are a bit confused by all this attention from her but are reluctant to say anything about it other than how "nice" and "friendly" my sister is.

I think this is all very inappropriate but am unsure how to address this. Being the eccentric she is, I am absolutely certain if I directly talk about this to my sister she will undoubtedly get extremely defensive, nasty and it will probably result in an even greater effort on her part to insert herself into my boyfriend's family. She did this before when her deceased friend's adult daughter finally told her to butt out. My sister actually helped cause a rift between the widower of her friend and this daughter, because of the way she was inserting herself into his life after the woman (her friend!) died.

Anyway, not sure how to proceed on this one. It's a weird case for sure! I've suggested to my boyfriend that he try and bring up her background when talking with his family so they can at least be aware of her meddling ways and cool down their response to her enthusiasm to develop her own relationship with his family. Any suggestions or people with similar experiences out there?