MIL and husband

I am married for 3.5 years. Both I and my husband work are in IT. I was 25 and he was 26 when we got married. My husband is very close with his mother. Sometimes their closeness makes me uncomfortable. I feel guilty for feeling and thinking this way. Trust me, I have tried to think that it is normal. But it never helped me. These are the below things which really made me uncomfortable and disturbed.
After our marriage, one day he was sitting and crying. I went to him and asked what happened. He told me that he was missing his mom and wants to see her. I felt weird for a grown up, newly married person to sit and cry like that. But I ignored.
After few weeks, my MIL came to stay with us. We stay with my BIL. Ours is 2 bed room house. We sleep in one room and BIL sleeps in another room. Before going to bed, my BIL called my MIL that he has done her mattress and told her to come and sleep when she is sleepy. For my surprise, she replied looking at my husband that she wants to sleep with him in our room. I was shocked. Next night also, she did the same. It continued for few more days. Still she asks him to sleep with her whenever she visits us.
One day when my MIL was supposed to travel back from our place next day. At mid night 3 or 4 o clock, I saw my husband was not there on our bed. I thought he might have gone to bath room or so. But when he did not come back after 30 mins also, I went and checked in the other room. My BIL sleeps on cot and my MIL sleeps on single mattress on floor. I was shocked to see him hugging her and sleeping with her on her mattress. He had put his arms over my MIL's bare stomach. I felt like puking. I checked with my husband about this next day. He casually told me that he was feeling bad that she is traveling next day and he missed sleeping with her as he sleeps with me now, so went and slept with her. I am never able to take that scene out of my head even 2 years after that incident.
My husband and MIL always go to room and lock themselves in room and never come out of room for a long time. I really don't know what they do inside that room.
She changes in front of him only. In summers, she just roams around in her blouse and petticoat inside her house. My MIL is just 50 years old and she still has proper figure and other body parts properly. She always calls my husband to help drape her saree even when my FIL is there at house.
When my husband is sitting on sofa and watching Tv, she just comes and sleep resting her head on his lap and start whispering something in his eyes and both of them start laughing whispering in each other's ears like how they do in movies.
I try to console myself that all these things are just because he is very close to his mom. Even I have younger brother. Though he is very much younger to my husband, he never behaves like that with my mother. Even my BIL does not behave like this with my MIL. They just have any other normal mother son relation. Only my husband and MIL have all these weird things. These things really make me uncomfortable and sick. Please tell me if it is normal or should I take any steps here.