Worried about controlling relationship

[Husbandsonfatherbrother]

I’m looking for advice on this situation. It’s my first time here so please be gentle.
DSIL(sister-in-law) has been in a new relationship for the last 7/8 months. She has just got engaged and is getting married in 2 months.
However we haven’t even meet her new partner yet!
SIL had just bought a house near her work and was settled in for the long haul. In the last 6 months she has given up her job and sold her house. Apparently they have “plans” but can’t tell us yet. Oh , and by the way , don’t tell anyone about what we’re doing.
SIL has deleted all photos of old partner and doesn’t see her friends from work or where she lives. She won’t even talk to them when they come to her house.
My DW and I are now extremely worried about her as we think she is being controlled.
What should we do?

I ought to add that DSIL is 5hr journey away so it’s not like we can pop round for a quick chat.

If she's doing what my niece did its all about the biological clock going off.
My niece was 33, great job, own home, single and desperate for marriage and children to head her way.
She took up with someone who has serious anxiety issues, no intention of working for a living and who was himself looking for someone to take care of him.
Five years on, two kids and run ragged with it all...she wonders why we didn't intervene and blames family for not 'telling', her.
We tried to but she wasn't listening.
She kept everything very secret too OP.

Hello this is a difficult one. Can you gently suggest to her its all going a bit fast and that she should slow down and take her time? Its tricky as its her life and she wont thank you if you go in too heavy.

Thanks @pallasathena, no biological clock issues As DSIL is in her 50’s and the new partner in their 60’s
@Lozzerbmc you’re right it’s very tricky as DW trying to get a chance to talk to her sister out of earshot of her partner but we are afraid her communications are being controlled. We’re very aware of treading gently as we think they might just disappear if they get wind of our suspicions